optimize node slices

[quasilyte]

Use a single struct type NodeSlice to wrap all
kinds of ast.Node slices; this allows us to have an allocation pool slice that can be used to
avoid allocations in most cases.

While some operations like NodeSlice.At become slower, other operations like NodeSlice.SliceInto are far more efficient: we can re-use memory even in re-slicing cases.

Benchmark results:

name                           old time/op    new time/op    delta
Match/failFast-8                  178ns ± 1%     182ns ± 1%   +2.18%  (p=0.000 n=10+10)
Match/failCall-8                  427ns ± 1%     414ns ± 1%   -3.00%  (p=0.000 n=10+9)
Match/failCallFast-8              176ns ± 0%     166ns ± 2%   -6.07%  (p=0.000 n=8+10)
Match/assign-8                    251ns ± 1%     245ns ± 0%   -2.29%  (p=0.000 n=10+7)
Match/assignMulti-8               874ns ± 2%     586ns ± 1%  -32.96%  (p=0.000 n=10+10)
Match/simpleLit-8                 109ns ± 1%     104ns ± 4%   -4.15%  (p=0.000 n=10+10)
Match/simpleBinaryOp-8            234ns ± 1%     235ns ± 4%     ~     (p=0.986 n=10+10)
Match/simpleSelectorExpr-8        326ns ± 1%     308ns ± 1%   -5.55%  (p=0.000 n=9+10)
Match/simpleCall-8                310ns ± 1%     296ns ± 1%   -4.25%  (p=0.000 n=10+10)
Match/selectorExpr-8              343ns ± 1%     339ns ± 1%   -1.10%  (p=0.004 n=10+9)
Match/sliceExpr-8                 261ns ± 1%     243ns ± 0%   -6.89%  (p=0.000 n=10+9)
Match/any-8                       181ns ± 1%     187ns ± 3%   +3.04%  (p=0.000 n=10+10)
Match/anyCall-8                   693ns ± 1%     514ns ± 1%  -25.87%  (p=0.000 n=10+10)
Match/ifStmt-8                    558ns ± 1%     399ns ± 1%  -28.51%  (p=0.000 n=10+10)
Match/optStmt1-8                  399ns ± 1%     375ns ± 1%   -6.13%  (p=0.000 n=10+10)
Match/optStmt2-8                  276ns ± 1%     247ns ± 1%  -10.64%  (p=0.000 n=10+9)
Match/namedOptStmt1-8            2.00µs ± 1%    1.61µs ± 1%  -19.18%  (p=0.000 n=9+10)
Match/namedOptStmt2-8            1.33µs ± 1%    1.06µs ± 1%  -20.07%  (p=0.000 n=10+10)
Match/branchStmt-8                139ns ± 1%     137ns ± 1%   -1.33%  (p=0.000 n=10+10)
Match/multiStmt-8                1.23µs ± 1%    0.98µs ± 1%  -20.06%  (p=0.000 n=9+10)
Match/multiExpr-8                1.33µs ± 1%    1.03µs ± 1%  -22.31%  (p=0.000 n=9+10)
Match/variadicCall-8              254ns ± 2%     252ns ± 0%   -0.79%  (p=0.003 n=10+7)
Match/capture1-8                  170ns ± 0%     167ns ± 1%   -2.24%  (p=0.000 n=9+9)
Match/capture2-8                  257ns ± 1%     250ns ± 1%   -2.75%  (p=0.000 n=10+10)
Match/capture8-8                  928ns ± 1%     919ns ± 1%   -0.95%  (p=0.000 n=10+10)
Match/capture2same-8              296ns ± 1%     283ns ± 0%   -4.43%  (p=0.000 n=10+9)
Match/capture8same-8             1.06µs ± 1%    1.03µs ± 1%   -3.41%  (p=0.000 n=9+10)
Match/captureBacktrackLeft-8     1.65µs ± 1%    1.10µs ± 1%  -32.96%  (p=0.000 n=10+8)
Match/captureBacktrackRight-8    1.08µs ± 2%    0.81µs ± 1%  -24.89%  (p=0.000 n=10+10)
Match/exprList-8                 2.62µs ± 1%    2.32µs ± 1%  -11.61%  (p=0.000 n=10+8)
[Geo mean]                        453ns          405ns       -10.66%

name                           old alloc/op   new alloc/op   delta
Match/failFast-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/failCall-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/failCallFast-8              32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/assign-8                    32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/assignMulti-8                192B ± 0%       96B ± 0%  -50.00%  (p=0.000 n=10+10)
Match/simpleLit-8                 32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/simpleBinaryOp-8            32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/simpleSelectorExpr-8        32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/simpleCall-8                32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/selectorExpr-8              32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/sliceExpr-8                 32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/any-8                       32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/anyCall-8                    144B ± 0%       96B ± 0%  -33.33%  (p=0.000 n=10+10)
Match/ifStmt-8                    80.0B ± 0%     32.0B ± 0%  -60.00%  (p=0.000 n=10+10)
Match/optStmt1-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/optStmt2-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/namedOptStmt1-8              344B ± 0%      224B ± 0%  -34.88%  (p=0.000 n=10+10)
Match/namedOptStmt2-8              168B ± 0%       96B ± 0%  -42.86%  (p=0.000 n=10+10)
Match/branchStmt-8                32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/multiStmt-8                  256B ± 0%      160B ± 0%  -37.50%  (p=0.000 n=10+10)
Match/multiExpr-8                  256B ± 0%      160B ± 0%  -37.50%  (p=0.000 n=10+10)
Match/variadicCall-8              32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/capture1-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/capture2-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/capture8-8                  32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/capture2same-8              32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/capture8same-8              32.0B ± 0%     32.0B ± 0%     ~     (all equal)
Match/captureBacktrackLeft-8       288B ± 0%       96B ± 0%  -66.67%  (p=0.000 n=10+10)
Match/captureBacktrackRight-8      168B ± 0%       96B ± 0%  -42.86%  (p=0.000 n=10+10)
Match/exprList-8                   256B ± 0%      160B ± 0%  -37.50%  (p=0.000 n=10+10)
[Geo mean]                        59.0B          48.2B       -18.32%