debuginfo: Began refactoring of composite type handling.
debuginfo: Renamed *reference-to-* test cases to *borrowed-*
debuginfo: Removed some misleading comments from test cases.
debuginfo: Refactored vec slice code to use the new infrastructure. Added test cases for vec slices.
debuginfo: Refactoring of composite type info generation done.

There is no more StructContext now. Better support for boxed vectors in there too.
debuginfo: Replaced vec::mapi with iterator version.
debuginfo: Added test cases for packed structs (/w drop)
debuginfo: Added support for c-style enums.
debuginfo: Support for tuple-style enums (WIP)
debuginfo: Better support for univariant tuple-style enums.
debuginfo: Added support for struct-style enums.
debuginfo: Fixes related to changed memory layout of unique allocations
Cleanup of ty::VariantInfo and related functions.
debuginfo: Major code cleanup in debuginfo.rs
debuginfo: Extended test suite with various tests for enums.
debuginfo: DI generation for enums uses adt::represent_type() now.
debuginfo: Fixed unique pointers to data containing managed pointers.
debuginfo: Added support for Option<T>-like enums.
debuginfo: Cleaned up style issues for pull request.
debuginfo: Adapted DI generation to new memory layout of unique vecs.