since the "precision" parameter means constant arithmetic is not
necessarily exact.

As requested by gri, within go/types, the local import name 'exact'
has been kept, to reduce the diff with the x/tools branch.  This may
be changed later.

Since the go/types.bash script was already obsolete, I added a comment
to this effect.

Tested with all.bash.

Change-Id: I45153688d9d8afa8384fb15229b0124c686059b4
Reviewed-on: https://go-review.googlesource.com/9242Reviewed-by: Rob Pike <r@golang.org>