Introduce mrb_ssize type for buffer size on memory; ref #4483

Previously, `mrb_int` was used as the type that represents the buffer size
on memory, but the sizes of `RString` and `RArray` exceed 6 words when
`MRB_INT64` is enabled on 32-bit CPU.

I don't think it is necessary to be able to represent the buffer size on
memory that exceeds the virtual address space. Therefore, for this purpose,
introduce `mrb_ssize` which doesn't exceed the sizes of `mrb_int` and
pointer.

I think all `mrb_int` used for this purpose should be changed to
`mrb_ssize`, but currently only the members of the structures (`RString`,
`mrb_shared_string`, `RArray` and `mrb_shared_array`) are changed.

include/mruby/string.h

@@ -23,9 +23,9 @@ struct RString {
   MRB_OBJECT_HEADER;
   union {
     struct {
-      mrb_int len;
+      mrb_ssize len;
       union {
-        mrb_int capa;
+        mrb_ssize capa;
         struct mrb_shared_string *shared;
         struct RString *fshared;
       } aux;
@@ -54,7 +54,7 @@ struct RStringEmbed {
     RSTR_SET_EMBED_LEN((s),(n));\
   }\
   else {\
-    (s)->as.heap.len = (mrb_int)(n);\
+    (s)->as.heap.len = (mrb_ssize)(n);\
   }\
 } while (0)
 #define RSTR_EMBED_PTR(s) (((struct RStringEmbed*)(s))->ary)