The line between ssa.Func and ssa.Config has blurred.
Concurrent compilation in the backend will require more precision.
This CL lays out an (aspirational) organization.
The implementation will come in follow-up CLs,
once the organization is settled.

ssa.Config holds basic compiler configuration,
mostly arch-specific information.
It is configured once, early on, and is readonly,
so it is safe for concurrent use.

ssa.Func is a single-shot object used for
compiling a single Func. It is not concurrency-safe
and not re-usable.

ssa.Cache is a multi-use object used to avoid
expensive allocations during compilation.
Each ssa.Func is given an ssa.Cache to use.
ssa.Cache is not concurrency-safe.

Change-Id: Id02809b6f3541541cac6c27bbb598834888ce1cc
Reviewed-on: https://go-review.googlesource.com/38160Reviewed-by: Keith Randall <khr@golang.org>