Optimize VariableCoercer

[kyri-petrou]

Seems that when we refactored the Validator to use ZPure we forgot(?) to do it with the VariableCoercer. Reality is that in most cases the variables will be a small / shallow object so this doesn't really provide much performance benefits for most users, except perhaps in cases where the inputs are deeply nested JSON files.

In this PR there are 2 different optimizations:

1. Use ZPure in favour of ZIO effects when recursing
2. Make context a by-name argument. This prevents us building the error context except when we need to fail the validation

Benchmarking results

Before:

[info] Benchmark                              Mode  Cnt      Score     Error  Units
[info] ValidationBenchmark.variableCoercer100   thrpt    5  14006.365 ± 180.586  ops/s
[info] ValidationBenchmark.variableCoercer1000  thrpt    5    518.925 ±  18.125  ops/s

After:

[info] Benchmark                                 Mode  Cnt      Score     Error  Units
[info] ValidationBenchmark.variableCoercer100   thrpt    5  87682.811 ± 661.300  ops/s
[info] ValidationBenchmark.variableCoercer1000  thrpt    5   8429.327 ±  28.180  ops/s

[ghostdogpr]

Nice!

[ghostdogpr]

This is super minor but how about extracting request.variables.getOrElse(Map.empty) into a variable so that hopefully this fits in one line and stays easier to read?

[kyri-petrou]

I was a bit wary of extracting them into a variable so we don't accidentally use them elsewhere but yeah the formatting was annoying me as well. Hopefully naming them unsafeVars (unsafeVariables didn't fit in a single line) will be clear enough that they're not to be used 😅