Variables can be declared and shadowing is supported, but modifying
existing variables via assignments was not available.

This meant that modifying a variable from a nested block was not
possible:

	{{ $v := "init" }}
	{{ if true }}
		{{ $v := "changed" }}
	{{ end }}
	v: {{ $v }} {{/* "init" */}}

Introduce the "=" assignment token, such that one can now do:

	{{ $v := "init" }}
	{{ if true }}
		{{ $v = "changed" }}
	{{ end }}
	v: {{ $v }} {{/* "changed" */}}

To avoid confusion, rename PipeNode.Decl to PipeNode.Vars, as the
variables may not always be declared after this change. Also change a
few other names to better reflect the added ambiguity of variables in
pipelines.

Modifying the text/template/parse package in a backwards incompatible
manner is acceptable, given that the package godoc clearly states that
it isn't intended for general use. It's the equivalent of an internal
package, back when internal packages didn't exist yet.

To make the changes to the parse package sit well with the cmd/api test,
update except.txt with the changes that we aren't worried about.

Fixes #10608.

Change-Id: I1f83a4297ee093fd45f9993cebb78fc9a9e81295
Reviewed-on: https://go-review.googlesource.com/84480
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>