html/template: fix quadratic performance with special tags

The current implementation of the tSpecialTagEnd function
is inefficient since it generates plenty of memory allocations
and converts the whole buffer to lowercase at each call.

If the number of special tags increases linearly with the
template size, the complexity becomes quadratic.

This CL provides an alternative implementation.
While the algorithm is probably still not optimal, it avoids
the quadratic behavior and the memory allocations.

benchmark                          old ns/op     new ns/op     delta
BenchmarkTemplateSpecialTags-4     19326431      532190        -97.25%

benchmark                          old allocs    new allocs    delta
BenchmarkTemplateSpecialTags-4     2650          190           -92.83%

benchmark                          old bytes     new bytes     delta
BenchmarkTemplateSpecialTags-4     4106460       46568         -98.87%

While we are there, make sure we respect the HTML tokenization algorithm.
An end tag needs to be followed by a space, tab, CR, FF, /, or > as described
in https://html.spec.whatwg.org/multipage/syntax.html#tokenization
Explicitly add this check.

Fixes #10605

Change-Id: Ia33ddee164ab608a69ac4183e16ec506bbeaa54c
Reviewed-on: https://go-review.googlesource.com/9502


Reviewed-by: Rob Pike <r@golang.org>