text/template: fix a couple of parse bugs around identifiers.

text/template: fix a couple of parse bugs around identifiers.
1) Poor error checking in variable declarations admitted
$x=2 or even $x%2.
2) Need white space or suitable termination character
after identifiers, so $x+2 doesn't parse, in case we want it
to mean something one day.
Number 2 in particular prevents mistakes that we will have
to honor later and so is necessary for Go 1.

Fixes #3270.
Fixes #3271.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5795073