expvar: Add benchmarks for perf sensitive operations

expvar: Add benchmarks for perf sensitive operations

These benchmarks are only for functions commonly used in loops. The
other functions are typically used for inspection or setup and thus are
not performance sensitive.

Change-Id: I8d0a0ba2d8234ecacb40fa3aa9077bf93c8fe89c
Reviewed-on: https://go-review.googlesource.com/3680
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>