auto merge of #7254 : Blei/rust/intrinsic-overhaul, r=cmr

This sets the `get_tydesc()` return type correctly and removes the intrinsic module. See #3730, #3475.

Update: this now also removes the unused shape fields in tydescs.

src/rt/rust_builtin.cpp

@@ -739,15 +739,6 @@ rust_task_deref(rust_task *task) {
     task->deref();
 }
 
-// Must call on rust stack.
-extern "C" CDECL void
-rust_call_tydesc_glue(void *root, size_t *tydesc, size_t glue_index) {
-    void (*glue_fn)(void *, void *, void *) =
-        (void (*)(void *, void *, void *))tydesc[glue_index];
-    if (glue_fn)
-        glue_fn(0, 0, root);
-}
-
 // Don't run on the Rust stack!
 extern "C" void
 rust_log_str(uint32_t level, const char *str, size_t size) {