mie: ctx is now used to query the type of a value

this allows value get_type callbacks to use mie_ctx to obtain mie_type pointers,
rather than having to define their own static versions of the type structs.

mie/type.c

@@ -1,6 +1,6 @@
 #include <mie/type.h>
 
-static struct mie_type *get_type(struct mie_value *v)
+static struct mie_type *get_type(struct mie_value *v, struct mie_ctx *ctx)
 {
 	return MIE_TYPE(v);
 }