reflect: optimize for maps with string keys

Over 80% of all Go map types use a string as the key.
The Go runtime already has a specialized implementation for such maps
in runtime/map_faststr.go. However, the Go reflection implementation
has not historically made use of that implementation.

This CL plumbs the appropriate logic to be accessible from Go reflection
so that it can benefit as well.

    name                            old time/op    new time/op    delta
    Map/StringKeys/MapIndex-4       4.65us ± 5%    2.95us ± 3%  -36.50%  (p=0.016 n=4+5)
    Map/StringKeys/SetMapIndex-4    7.47us ± 5%    5.27us ± 2%  -29.40%  (p=0.008 n=5+5)
    Map/Uint64Keys/MapIndex-4       3.79us ± 3%    3.75us ± 2%     ~     (p=0.548 n=5+5)
    Map/Uint64Keys/SetMapIndex-4    6.13us ± 3%    6.09us ± 1%     ~     (p=0.746 n=5+5)

Change-Id: I5495d48948d8caf2d004a03ae1820ab5f8729670
Reviewed-on: https://go-review.googlesource.com/c/go/+/345486


Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>