go/token: allocate fewer times at init time

go/token has had a global "keywords" map filled at init time for years.
Overall, the package's init time cost is small, as per GODEBUG=inittrace=1:

	init go/token @0.51 ms, 0.004 ms clock, 1776 bytes, 5 allocs
	init go/token @0.44 ms, 0.003 ms clock, 1776 bytes, 5 allocs
	init go/token @0.45 ms, 0.003 ms clock, 1568 bytes, 4 allocs

However, adding the map size hint does help with the allocations:

	init go/token @0.45 ms, 0.002 ms clock, 944 bytes, 2 allocs
	init go/token @0.46 ms, 0.002 ms clock, 944 bytes, 2 allocs
	init go/token @0.55 ms, 0.003 ms clock, 1152 bytes, 3 allocs

Three samples are rather unscientific, and the clock time is basically
unchanged, but we might as well reduce the allocs.

Change-Id: I48121a4cea4113d991882e32f274d7b7736800dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/391094


Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>