Cleanup and type fixes for impregnation

Partly from comment chain on !8041 (merged)

1. Use a constructor to build fetus objects and get rid of the external type definition for FC.Fetus.
2. Significantly simplify fetusSplit. Remove identical and splitted fetus tags; twinID and a smarter loop can do all the work.
3. WombImpregnateClone will do the necessary genepool lookup itself, instead of forcing the caller to do it, and will set missing parents only once.
4. generateGenetics now takes the fetus name directly, instead of building it from the descriptively named numeric parameter x, and loses a couple of unnecessary subfunctions that just return their parameter.
5. Fix some type annotations (make PlayerState and SlaveState slightly more compatible, add some missing annotations, etc).