runtime/internal/maps: optimize long string keys for small maps

runtime/internal/maps: optimize long string keys for small maps

For large strings, do a quick equality check on all the slots.
Only if more than one passes the quick equality check do we
resort to hashing.

                               │    baseline    │             experiment              │
                               │     sec/op     │   sec/op     vs base                │
MegMap-24                        16609.50n ± 1%   13.91n ± 3%  -99.92% (p=0.000 n=10)
MegOneMap-24                     16655.00n ± 0%   12.27n ± 1%  -99.93% (p=0.000 n=10)
MegEqMap-24                         41.31µ ± 1%   25.03µ ± 1%  -39.40% (p=0.000 n=10)
MegEmptyMap-24                      2.034n ± 0%   2.027n ± 2%        ~ (p=0.541 n=10)
MegEmptyMapWithInterfaceKey-24      5.931n ± 2%   5.599n ± 1%   -5.60% (p=0.000 n=10)
MapStringKeysEight_16-24            8.473n ± 7%   8.224n ± 5%        ~ (p=0.315 n=10)
MapStringKeysEight_32-24            8.441n ± 2%   8.147n ± 1%   -3.48% (p=0.002 n=10)
MapStringKeysEight_64-24            8.769n ± 1%   8.517n ± 1%   -2.87% (p=0.000 n=10)
MapStringKeysEight_128-24           10.73n ± 4%   13.57n ± 8%  +26.57% (p=0.000 n=10)
MapStringKeysEight_256-24           12.97n ± 2%   14.35n ± 4%  +10.64% (p=0.001 n=10)
MapStringKeysEight_1M-24         17359.50n ± 3%   13.92n ± 4%  -99.92% (p=0.000 n=10)

Change-Id: I4cc2ea4edab12a4b03236de626c7bcf0f96b6cc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/625905
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>