runtime: move mapaccess1 and mapassign to internal/runtime/maps

runtime: move mapaccess1 and mapassign to internal/runtime/maps

This enables manual inlining Map.Get/table.getWithoutKey to create a
simple fast path with no calls.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: Ic208dd4c02c7554f312b85b5fadccaf82b23545c
Reviewed-on: https://go-review.googlesource.com/c/go/+/616455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>