'Did you mean' for variables missing on one side of an or-pattern
simplify spellchecking of labels in typecore.ml:

By computing the list of valid names at Wrong_name raising point, we
can remove the logic to reconstruct the set of valid names to draw
suggestion from after the fact:
- no spellchecking-related code in the NameChoice functor
- seamlessly extends to fields of inline records (were not spellchecked before)

I made the type of the "wrong name" in the Wrong_name exception more
precise, from Longident.t to string. The rationale is that:
- we only raise this exception when the longident happens to be
  a (Lident str)
- the list of valid names is a list of field or variant names; those
  are strings rather than idents
typecore.ml: minor refactoring
'Did you mean' for unbound method names