Don't alias #\Bell to #\Bel.

Unicode character U+1F514 has the name BELL.

Patch by Krzysztof Drewniak, lp#1319452.
Fix CAS SYMBOL-VALUE for locally special variables.

The case-form of the expansion was wrong, and the read-form used
SYMBOL-GLOBAL-VALUE.

Fixes lp#1098355.
Optimize (- (the (integer 0 2^63) x)) on x86-64.

2^63 is the only unsigned-byte-64 value which fits (signed-byte 64)
when negated. Add FAST-NEGATE/UNSIGNED VOP.