Add an option for converting usize/isize into size_t/ptrdiff_t

tests/rust/size-types.rs

@@ -0,0 +1,15 @@
+type Usize = usize;
+type Isize = isize;
+
+#[repr(usize)]
+enum UE {
+    UV,
+}
+
+#[repr(isize)]
+enum IE {
+    IV,
+}
+
+#[no_mangle]
+pub extern "C" fn root(_: Usize, _: Isize, _: UE, _: IE) {}

tests/rust/size-types.toml

@@ -0,0 +1 @@
+usize_is_size_t = true