Use runtime.CallersFrames

[bufdev]

Just doing #354

[prashantv]

Can you post a summary of any differences in the benchmark results from switching to CallerFrames? Just curious what kind of performance penalty we're paying (if any).

[akshayjshah]

The relevant benchmark here is BenchmarkStackField.

[bufdev]

# master
BenchmarkStackField-8   	 1000000	      1032 ns/op	     449 B/op	       2 allocs/op
# runtime-frames
BenchmarkStackField-8   	 1000000	      1008 ns/op	     449 B/op	       2 allocs/op

no change it seems like

[akshayjshah]

The escape analyzer & inlining have our back here - even though CallerFrames returns a reference, the func gets inlined so it doesn't escape and so we're not penalized.