reflect: cache call frames

reflect: cache call frames

Call frame allocations can account for significant portion
of all allocations in a program, if call is executed
in an inner loop (e.g. to process every line in a log).
On the other hand, the allocation is easy to remove
using sync.Pool since the allocation is strictly scoped.

benchmark           old ns/op     new ns/op     delta
BenchmarkCall       634           338           -46.69%
BenchmarkCall-4     496           167           -66.33%

benchmark           old allocs     new allocs     delta
BenchmarkCall       1              0              -100.00%
BenchmarkCall-4     1              0              -100.00%

Update #7818

Change-Id: Icf60cce0a9be82e6171f0c0bd80dee2393db54a7
Reviewed-on: https://go-review.googlesource.com/1954
Reviewed-by: Keith Randall <khr@golang.org>