Fix misplaced LOOP declarations.

Split declarations into two kinds, descrtructuring and normal.

Fixes lp#700538.
Simplify LOOP.

Simplify LOOP-SEQUENCER. And also correct a problem in the previous
commit.
Optimize LOOP REPEAT.

Declare the iteration variable when the repeat number is known.
Check more thoroughly for duplicate variables in LOOP.

Fixes lp#645534.
Get rid of unused warnings for LOOP with multiple MINIMIZE.

Avoid wrapping the loop with duplicate with-minimax-value forms.

Fixes lp#613876.
Fix destructuring unsupplied arguments in LOOP.

LOOP uses destructuring-bind, so just add &optional at the beginning
of the lambda-list.

Fixes lp#695286.