Do implicit unary conversions in one function.

This is a part of the effort to fix 8cc's implicit type conversions.
I believe this patch makes 8cc's behavior to close to the standard.

The new function, conv(), does the following.

 - convert an array of T to a pointer to T
 - convert a functiond designator to a pointer to the function
 - do the usual integer conversions
Do a better job of the usual arithmetic conversions.