Make keys be {int:str} when constructing categorical fields

[atbenmurray]

Keys are currently passed to categorical field constructors as {str:int} when constructing strings and then inverted. This creates complexity throughout the rest of the categorical field code and should be limited to data import only.

The solution was actually to just convert dictionaries of {str/byte: int} or {byte/str: int} to {int:str} as a canonical format, which makes the problem essentially not a problem