enum map
========

- a map from enum -> values
- should support both open and closed enums
- should support absent values in case of closed enums
- should support data in constant data section


two types:
    type CompleteEnumMap    (all values are known to exist. can only be used with closed enums)
    type IncompleteEnumMap  (some values might be absent)

example use case:
    data String prefix = "/usr/local"
    data IncompleteEnumMap<enum PrefixLocation> prefixes = {
        .exec_prefix = "/usr/local/x86_64-linux-gnu"
    }