text/template: efficient reporting of line numbers

text/template: efficient reporting of line numbers

Instead of scanning the text to count newlines, which is n², keep track as we go
and store the line number in the token.

benchmark                 old ns/op      new ns/op     delta
BenchmarkParseLarge-4     1589721293     38783310      -97.56%

Fixes #17851

Change-Id: Ieaf89a35e371b405ad92e38baa1e3fa98d18cfb4
Reviewed-on: https://go-review.googlesource.com/32923
Reviewed-by: Robert Griesemer <gri@golang.org>