Riemann mapping theorem

This file contains partial results towards Riemann Mapping Theorem. A complete proof is available at https://github.com/leanprover-community/mathlib4/pull/33505, though it may fail to compile with the latest Mathlib.

It is being brought up to Mathlib code standards and merged in a series of smaller PRs. For now, all lemmas in this file are strictly weaker than the final theorem, so they're private.