changing function slots should update compiler properties

[Dave Love]

Functions which update symbols' function slots should remove compiler
properties first.  E.g., if you redefine something with `defun' that
previously was defined with `defsubst', it currently maintains the
`byte-compile-inline-expand' property.

Also I think that `defalias' should copy compiler properties where
appropriate.  E.g. after `(defalias 'plus '+)', I think calls of
`plus' should compile like calls of `+'.  I'm not completely sure it's
safe, though.