Other than catching an error case that was missed before, this
CL introduces no changes to the template language or API.

For simplicity, templates use spaces as argument separators.
This means that spaces are significant: .x .y is not the same as .x.y.
In the existing code, these cases are discriminated by the lexer,
but that means for instance that (a b).x cannot be distinguished
from (a b) .x, which is lousy. Although that syntax is not
supported yet, we want to support it and this CL is a necessary
step.

This CL emits a "space" token (actually a run of spaces) from
the lexer so the parser can discriminate these cases. It therefore
fixes a couple of undisclosed bugs ("hi".x is now an error) but
doesn't otherwise change the language. Later CLs will amend
the grammar to make .X a proper operator.

There is one unpleasantness: With space a token, three-token
lookahead is now required when parsing variable declarations
to discriminate them from plain variable references. Otherwise
the change isn't bad.

The CL also moves the debugging print code out of the lexer
into the test, which is the only place it's needed or useful.

Step towards resolving issue 3999.
It still remains to move field chaining out of the lexer
and into the parser and make field access an operator.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6492054