Fix several errors: The address operator on functions (resulting in a

Fix several errors: The address operator on functions (resulting in a
pointer-to-function type) was not possible. It is redundant but allowed.
A cast with a function on the right side resulted in an internal compiler
error, since the implicit cast from function to pointer-to-function was
missing in this case.
An assignment of a function to a void pointer was not possible because the
same implicit conversion was missing.

git-svn-id: svn://svn.cc65.org/cc65/trunk@482 b7a2c559-68d2-44c3-8de9-860c34a00d81