LOOP: fix destructuring and of-type.

Presence of NIL in the destructuring spec confused by NIL variables.
First, initialize ignored variables to the appropriate init-type, i.e.
0.0 for float, etc.
Then, do not bother with considering NIL from the last CDR of a proper
list as a variable.

Fixes lp#1322923.
Remove most META-SC stuff.

The backends will be cleaned up in a second commit.
Remove META-SC-NUMBER-OR-LOSE