Added a Triple structure to parse and manage target triples.
Add implementations of FromStr for Os && Architecture. Additionally
added some deriving types to both.
Store the parse target triple in Config.
Added convience functions to access the OS, arch, && the full triple.
Removed a duplication of ```syntax::abi::Os```.
Refactor retrieving the target && host library paths in the sysroot.
Impl ```std::clone::Clone``` for FileSearch.
New target triple crate metadata.
Filter crates based on the crate's target triple, if present.
If cross compiling, use rustc's set of crates while resolving syntax extension imports.
Refactor ```rustc::back::rpath``` to use Triple.