core: Use casting functions.
Silence all casting errors by using casting functions.
* src/cc.c (cast_charp_to_scmp, cast_charp_to_scmpp,
cast_voidp_to_charp, cast_scmp_to_long, cast_scmp_to_charp): New function.
* src/m2.c (cast_charp_to_scmp, cast_charp_to_scmpp,
cast_voidp_to_charp, cast_scmp_to_long, cast_scmp_to_charp): New function.
* include/mes/mes.h: Declare them.
(struct scm): Add car_value, cdr_value, function.
(g_continuations): Change to long.
* src/mes.c: Silence all casting errors by using casting functions.