mie: implemented array value type; restructure const value structures

there are now separate structs for all const types (int, string, etc),
rather than a single mie_const union.

mie/const.c

@@ -2,20 +2,12 @@
 #include <stdlib.h>
 #include <string.h>
 
-struct mie_const *mie_const_create(struct mie_type *type)
+void mie_const_init(struct mie_const *c, struct mie_type *type)
 {
-	struct mie_const *c = malloc(sizeof *c);
-	if (!c) {
-		return NULL;
-	}
-
 	memset(c, 0x0, sizeof *c);
-
 	mie_value_init(&c->c_base, MIE_VALUE_CONST);
 
 	c->c_type = type;
-
-	return c;
 }
 
 static struct mie_type *get_type(struct mie_value *v)