Client refactor

[kegsay]

• Remove Func from MustDoFunc and DoFunc. They were called that to differentiate from the old-style Do and MustDo which are no more.
• Move sync functions to a new file.
• Move registration/login functions to a new file.
• Move /internal/client to /client.
• Signal unsafe functions.
• Add more helper functions where their usage is proven in tests.
• Use TestLike, an interface to represent testing.T to allow the client to be used outside of tests e.g benchmarks.

Slightly not client related:

• Add federation.Event instead of abusing b.Event for creating events over federation. Makes this much clearer and fixes #381

Fixes https://github.com/matrix-org/complement/issues/526 , https://github.com/matrix-org/complement/issues/381

Partly addresses https://github.com/matrix-org/complement/issues/226