Implement syntax checks for pocket universes

Implement syntax checks for pocket universes

Summary:
Implement some local (per file) check for Pocket Universes syntax:

- In a class, each `enum` definition must be uniquely named
- In a single enum definition, each case type must be uniquely named
- In a single enum definition, each case expr must be uniquely named
- In a single enum definition, each atom mapping must be uniquely named
- In a single atom mapping each type definition must be uniquely named
- In a single atom mapping each expr definition must be uniquely named

Things we don't test here:
- that each definition is complete (all symbols are defined exactly one,
  it requires information for the parent).
- that there is not incompatibilities with the parent's definitions.

It will be done during typing or decl.

Reviewed By: CatherineGasnier

Differential Revision: D14621055

fbshipit-source-id: 719f6a52a1e45fb6581b40cfd47b29ada0f676be