add .gitignore
remove deprecated SB-C::MERGE-TAIL-CALLS declaration

... to fix a STYLE-WARNING.
unbreak on SBCL versions later than 1.1.18

GET-MACRO-CHARACTER returns closures for dispatch macro characters.
Add a kludge to treat these closures equivalent when resolving
readtable conflicts.
update %MAKE-READTABLE-ITERATOR for SBCL

Followup to SBCL commits c29b4b7 and fa0c056.

Patch from Douglas Katzman.