Added c-cast, struct member peeks, param syntax checking.

Added c-cast, struct member peeks, param syntax checking.

* src/ffi/cdecls.scm (valid-param): Check that the param name does not
contain `-', nor any other non-C identifier chars.  These names go
into the generated .c files.

* src/ffi/syntax.scm (expand-peek): Allow peeks at struct members to
create or set an alien, just as peeking an array member does already.

* src/ffi/ffi.scm (c-cast): New.  Basically set-%alien/ctype! with a
convenient return value.

(alien/address, copy-alien-address!, alien-null?, alien-null!, alien=?):
Declare these as integrable operators, not via define-integrable.
Their arguments are referenced multiple times.

* src/runtime/runtime.pkg (runtime ffi): Export c-cast to ().