Allow "matchers" for types such that cases can be labeled with identifiers rather than integers

[sdleffler]

Currently, the integer identifiers used for indexing cases in the case! macro correspond to type parameters passed to the Case<const N: usize> trait. This makes it a bit inconvenient to name the case identifiers with anything other than usize literals. But there is a way we can get around this, by generating macros that convert identifiers into usize tokens:

mod Option {
    #[macro_export]
    macro_rules! Option_matcher {
        (Some) => { 0 };
        (None) => { 1 };
    }
}

Unfortunately there's a large snag with this approach; macros in "path-based" scope will ALWAYS live in the crate root where they are defined. As a result, punning modules and types and getting the kind of scope punning we want does not work. Some kind of bespoke name mangling inside the case! macro might work; or, we might have to make the concept of "matcher" or "case address"/"indexing" macros more first-class.