debug/gosym: add funcTab abstraction

debug/gosym: add funcTab abstraction

This clarifies the existing code and makes modifications easier.

name            old time/op    new time/op    delta
115/LineToPC-8    58.6µs ± 3%    56.4µs ± 3%  -3.80%  (p=0.000 n=15+15)
115/PCToLine-8     194ns ± 2%     188ns ± 2%  -3.31%  (p=0.000 n=15+15)

Change-Id: Iafdf57af93d5e3c145965c32e0227e37c69ab017
Reviewed-on: https://go-review.googlesource.com/c/go/+/353880
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>