A constructive Cantor–Schröder–Bernstein theorem

[fredrik-bakke]

Formalizes a fine-grained analysis of the construction used for the Cantor-Schröder-Bernstein-Escardó theorem, and uses this deconstruction to give a series of generalizations of the theorem. Most importantly,

• If WLPO is true, andA and B mutually embed by decidable embeddings then A and B are equivalent.

[fredrik-bakke]

Invoking our self-review policy, I will review and merge this PR in a couple of days.

[fredrik-bakke]

Just to be clear, this PR does not affect the licensing issues laid out in #1635 and should be considered independently of that.

[fredrik-bakke]

This concludes my self-review, and this will be merged shortly. Any issues identified later can also be fixed later.