Bikeshed alt check

This is a compendium of match check expressions in libraries and rustc. My goal is to figure out how many there are of each kinds. I'm guessing there will be the following kinds of match checks:

1. User input (we can't really do anything about this except add an error case. Time format strings are an example; anything involving parsing, basically.)

• std::time::strftime
• test cases in core::comm, core::run, core::str, std::ebml, std::getopts
• serialization::deserialize_option
• code generated by autoserialize

1. Subsets of enums (hopefully will be addressed in the future with case classes / refinement types)

• trans::base::trans_lval (type of a unary operation -- would be great if typeck could produce something with a refined type...)
• std::list::head (great argument for refinements)

1. Difficult stuff (example: match len % 3 { ... where you know where will only be three cases. We will probably never have a fancy enough type system to make this exhaustive)

• std::base64 impl of to_base64 for ~[u8]

1. Easily rewritten (for example, when you only handle a specific variant and you can just pass its components instead)

• trans::base::make_mono_id (a vec of mono_ids all constructed with mono_precise gets consumed immediately)
• trans::base::llvm_type_name (only called on obvious enum and class types, so just pass the def_id and substs)
• core::bool::from_str (already had a _ case!)
• code dealing with expr_log things (made log_level an enum instead of an int)

1. Addressed by non-trivial refactoring (see send_map for an example)

• core::send_map (several)
• matches on ast::expr_loop_body and ast::expr_do_body things (refactoring the AST to eliminate junk -- this could have been also addressed with subsets-of-enums)

1. Results of metadata lookup (not much we can do here except add an error case, as in 1.)

• trans::base::monomorphic_fn::maybe_instantiate_inline

1. Not sure, because there's no documentation of why it's safe to omit cases

• syntax::attr::find_linkage_metas