debug/gosym: bulk allocate Syms

name                old time/op    new time/op    delta
115/NewLineTable-8    80.7ns ± 1%    81.7ns ± 3%     ~     (p=0.408 n=10+8)
115/NewTable-8        76.3µs ± 2%    63.3µs ± 3%  -17.03%  (p=0.000 n=9+9)
115/LineToPC-8        56.8µs ± 2%    56.8µs ± 3%     ~     (p=0.905 n=10+9)
115/PCToLine-8         252ns ± 1%     256ns ± 5%     ~     (p=0.183 n=10+10)

name                old alloc/op   new alloc/op   delta
115/NewLineTable-8      384B ± 0%      384B ± 0%     ~     (all equal)
115/NewTable-8         164kB ± 0%     172kB ± 0%   +4.97%  (p=0.000 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%     0.01k ± 0%  -98.75%  (p=0.000 n=10+10)
115/LineToPC-8          0.00           0.00          ~     (all equal)
115/PCToLine-8          0.00           0.00          ~     (all equal)

Change-Id: Ibfe6c0cb86737dee8209cb1f58f5b796aa7b30aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/353134


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>