runtime: move map fast functions into type specific files

Overall code is unchanged.

The functions for different types (32, 64, str) of map fast routines
are collected in map_fast.go that has grown to ~1300 lines.

Moving the functions for each map fast type into a separate file
allows for an easier overview and navigation within the map code.

Change-Id: Ic09e4212f9025a66a10b11ef8dac23ad49d1d5ae
Reviewed-on: https://go-review.googlesource.com/90335
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>