runtime: move userArenaHeapBitsSetType into mbitmap.go
This will make the upcoming GOEXPERIMENT easier to implement, since this
function relies on a lot of heap bitmap internals.
Change-Id: I2ab76e928e7bfd383dcdb5bfe72c9b23c2002a4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/537979 Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>