configure: Require one of wget or curl.
auto merge of #7498 : luqmana/rust/cow, r=catamorphism

Either one works and wget is easier to setup on windows.
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)