Fix get_tydesc() return type

This fixes part of #3730, but not all.
Also changes the TyDesc struct to be equivalent with the generated
code, with the hope that the above issue may one day be closed for good,
i.e. that the TyDesc type can completely be specified in the Rust
sources and not be generated.
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