feat: prove T297, countably many continuous self-maps implies countable

[kisonecat]

Adds T297: a space with countably many continuous self-maps is countable.

Constant maps are continuous, and distinct points give distinct constant maps, so X injects
into C(X, X). Recovering the point from the constant map needs a point to evaluate at, so
the empty space is handled separately — where the conclusion is immediate.

This is the proof π-base records: each constant map is a continuous map.

Closes #551.