Might as well tell the runtime how large the map is going to be.
This avoids grow work and allocations while the map is being built.

Will wait for 1.8.

Fixes #15880
Fixes #16279

Change-Id: I377e3e5ec1e2e76ea2a50cc00810adda20ad0e79
Reviewed-on: https://go-review.googlesource.com/23558
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>