This is a rewrite of EctoEnum, with a much simpler goal.
EctoEnum assumes its storage is a dumb integer-typed DB field, and thus must create stable mappings between the atoms you provide it, and the integer keys they must serialize to.
EctoHomoiconicEnum assumes its storage is either a text-typed column, or a literal enum column with string serialization semantics (like a Postgres ENUM.) Such columns will both accept and return the string representations of the enum's values, so EctoHomoiconicEnum has no need to maintain mappings for its keys.
Instead, EctoHomoiconicEnum simply acts to validate that your keys are members of the values set you specify.