1.0.48.28: make TRULY-THE macroexpandable

1.0.48.28: make TRULY-THE macroexpandable

  lp#771673

  * MACRO-FUNCTION no longer checks :FUNCTION :KIND, allowing us to have
    special operators with macro-expansions in the first place.

  * Add %MACROEXPAND and %MACROEXPAND which are careful not to expand special
    forms, and use them in place of SB!XC:MACROEXPAND.

  * Set the :FUNCTION :MACRO-FUNCTION into an expander that just converts
    it to THE. This only happens when someone explicitly calls MACROEXPAND
    or uses MACRO-FUNCTION directly -- never in the compiler.

  * Also add a SETF-expander.