runtime: add specialized copies of growWork and evacuate

The newly added routines are exact copies of the generic routines,
except for the function names and that growWork_fastX calls evacuate_fastX.

Actual optimization will happen in subsequent CLs.
This is intended to ease reviewing.

Change-Id: I52ef7dd40b2bdfc9cba2496544c0604e6e71cf7f
Reviewed-on: https://go-review.googlesource.com/59130
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>