Remove redundant imports in lib (rustc doesn't like 'std' as a synonym for root within std.rc anyway)

It now uses a string writer to also fill in for
middle.ty.ast_ty_to_str

For use by pretty-printer. string_writer API is a bit silly
right now, feel free to suggest a cleaner way to do this.

rustc: Make test/run-pass/generic-tag-alt.rs valid Rust; it was passing a dynamically-sized value by value instead of by alias

rustc: Don't try to load dynamically-sized types when translating tag variants. Add a test case for this.

rustc: Instantiate tag patterns with the type parameters supplied by the programmer. size-and-align.rs typechecks now.

rustc: Factor the type parameter and type lookup out of instantiate_path(), as patterns need different logic

rustc: Have trans get the type parameters from the annotation instead of trying to deduce them itself. Un-XFAIL generic-fn-twice.rs.

This reverts commit 55c80e76.

in each "translation unit".

rustc: Rework type substitution to pair up type parameter IDs with the supplied type parameter instantiations explicitly

rustc: Store the number of type parameters per item, which will be needed to get unused type params working