internal/abi,runtime: refactor map constants into one place

internal/abi,runtime: refactor map constants into one place

Previously TryBot-tested with bucket bits = 4.
Also tested locally with bucket bits = 5.
This makes it much easier to change the size of map
buckets, and hopefully provides pointers to all the
code that in some way depends on details of map layout.

Change-Id: I9f6669d1eadd02f182d0bc3f959dc5f385fa1683
Reviewed-on: https://go-review.googlesource.com/c/go/+/462115
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Austin Clements <austin@google.com>