Ideal constants in the template package are a little different from Go.
This is a case that slipped through the cracks: A huge integer number
was accepted as a floating-point number, but this loses precision
and is confusing. Also, the code in the template package (as opposed
to the parse package) wasn't expecting it.

Root this out at the source: If an integer doesn't fit an int64 or uint64,
complain right away.

Change-Id: I375621e6f5333c4d53f053a3c84a9af051711b7a
Reviewed-on: https://go-review.googlesource.com/9651Reviewed-by: Russ Cox <rsc@golang.org>