Remove intrinsic module

To achieve this, the following changes were made:
* Move TyDesc, TyVisitor and Opaque to std::unstable::intrinsics
* Convert TyDesc, TyVisitor and Opaque to lang items instead of specially
  handling the intrinsics module
* Removed TypeDesc, FreeGlue and get_type_desc() from sys

Fixes #3475.
Fix and reenable the reflect-visit-type test
Remove unused shape fields from typedescs
Small cleanups
Remove rust_call_tydesc_glue

Towards #4812. Also includes some minor cleanups.
syntax: Remove unused tokens

TyDesc, TyVisitor and intrinsic are not used anymore.