runtime: rename mapiterinit and mapiternext

mapiterinit allows external linkname. These users must allocate their
own iter struct for initialization by mapiterinit. Since the type is
unexported, they also must define the struct themselves. As a result,
they of course define the struct matching the old hiter definition (in
map_noswiss.go).

The old definition is smaller on 32-bit platforms. On those platforms,
mapiternext will clobber memory outside of the caller's allocation.

On all platforms, the pointer layout between the old hiter and new
maps.Iter does not match. Thus the GC may miss pointers and free
reachable objects early, or it may see non-pointers that look like heap
pointers and throw due to invalid references to free objects.

To avoid these issues, we must keep mapiterinit and mapiternext with the
old hiter definition. The most straightforward way to do this is to use
mapiterinit and mapiternext as a compatibility layer between the old and
new iter types.

The first step to that is to move normal map use off of these functions,
which is what this CL does.

Introduce new mapIterStart and mapIterNext functions that replace the
former functions everywhere in the toolchain. These have the same
behavior as the old functions.

This CL temporarily makes the old functions throw to ensure we don't
have hidden dependencies on them. We cannot remove them entirely because
GOEXPERIMENT=noswissmap still uses the old names, and internal/goobj
requires all builtins to exist regardless of GOEXPERIMENT. The next CL
will introduce the compatibility layer.

I want to avoid using linkname between runtime and reflect, as that
would also allow external linknames. So mapIterStart and mapIterNext are
duplicated in reflect, which can be done trivially, as it imports
internal/runtime/maps.

For #71408.

Change-Id: I6a6a636c6d4bd1392618c67ca648d3f061afe669
Reviewed-on: https://go-review.googlesource.com/c/go/+/643898


Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>