math/big: make NewInt inlineable and zero allocation

math/big: make NewInt inlineable and zero allocation

Mark the assembly routines as not escaping their arguments.

Add a special case to NewInt that, when inlined, can do all
of its allocations (a big.Int and a [1]Word) on the stack.

Update #29951

Change-Id: I9bd38c262eb97df98c0ed9874da7daac381243ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/411254
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>