spec: clarify variable declaration type rules

Not a language change.

Several inaccuracies were fixed:

1) A variable declaration may declare more than just one
variable.

2) Variable initialization follows the rules of assignments,
including n:1 assignments. The existing wording implied a 1:1
or n:n rule and generally was somewhat unspecific.

3) The rules for variable declarations with no types and
untyped initialization expressions had minor holes (issue 8088).

4) Clarified the special cases of assignments of untyped values
(we don't just have untyped constants, but also untyped bools,
e.g. from comparisons). The new wording is more direct.

To that end, introduced the notion of an untyped constant's
"default type" so that the same concept doesn't have to be
repeatedly introduced.

Fixes issue 8088.

LGTM=iant, r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/142320043

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