Why track mapping with generics?

[KodrAus]

I think it's worth calling out why this library is built the way it is. The main goal of keeping mapping on fields is so that in the future they could be used to help construct strongly-typed query DSL queries. That's not an immediate goal of elastic, but by tracking types as traits with generic mapping, we can thread things like formats through to range queries to make sure date expressions are formatted properly.