Ticket40046: Define and use CONST_TO_FOO_CONN()

This branch adds a set of const-to-const casting functions for channels and connections, to mirror the ones we already have for circuits.

I've structured three of these commits as a squash! series, since the first is automated and the next two revert parts of it by hand.

See the Github PR at https://github.com/torproject/tor/pull/2008 for CI.

Closes #40046 (closed)