debug/gosym: refactor handling of funcdata

debug/gosym: refactor handling of funcdata

We do a bunch of manual offset calculations everywhere.
Add a bit of type safety and some helpers.
In addition to making the code clearer and providing a place
to hang some documentation, it also makes upcoming changes easier.

name                old time/op    new time/op    delta
115/NewLineTable-8    79.9ns ± 1%    90.2ns ±23%    ~     (p=0.234 n=9+10)
115/NewTable-8        72.0µs ± 1%    73.4µs ± 1%  +1.96%  (p=0.000 n=8+8)
115/LineToPC-8        53.3µs ± 1%    54.4µs ± 1%  +2.02%  (p=0.000 n=10+10)
115/PCToLine-8         249ns ± 0%     249ns ± 2%    ~     (p=0.147 n=9+10)

name                old alloc/op   new alloc/op   delta
115/NewLineTable-8      384B ± 0%      384B ± 0%    ~     (all equal)
115/NewTable-8         164kB ± 0%     164kB ± 0%    ~     (p=0.610 n=10+10)
115/LineToPC-8         0.00B          0.00B         ~     (all equal)
115/PCToLine-8         0.00B          0.00B         ~     (all equal)

name                old allocs/op  new allocs/op  delta
115/NewLineTable-8      3.00 ± 0%      3.00 ± 0%    ~     (all equal)
115/NewTable-8         1.04k ± 0%     1.04k ± 0%    ~     (all equal)
115/LineToPC-8          0.00           0.00         ~     (all equal)
115/PCToLine-8          0.00           0.00         ~     (all equal)

Change-Id: If357dce5ae4277e6ddc6d90ba6b5b83e470b9121
Reviewed-on: https://go-review.googlesource.com/c/go/+/352951
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>