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
• read_vtable_origin in astencode

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)
• encoder::purity_static_method_family (we currently can't encode that static methods can never have purity extern_fn)

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)
• matching on optimization levels in back::link and driver::driver (make optimization level an enum)
• Log levels in expr_log (make it an enum rather than a uint)
• syntax::attrs::find_linkage_metas (addressed by inlining find_linkage_attrs, which didn't really need to be a separate function, into find_linkage_metas)

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)
• lint::check_fn -- there's currently no constraint that a function must have a function type (in the type context)
• only some combinations of types are supported in a cast in middle::trans::consts

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
• decoder::item_to_def_like (the Variant case; item_parent_item returns an option)
• get_trait_methods (doc for method might have an item_family that's not a purity value)
• decoder::family_to_visibility (similar)

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

• syntax::attr::find_linkage_metas