Add support for more complex Action types

[alice-i-cecile]

This is a complex PR: it completely refactors our internals in order to allow support for more complex action types and generally reduce internal jank.

Namely, it:

1. Removes Actionlike::variants and other methods that assume a finite ordered list of discrete actions.
2. Passes action types by reference rather than value.
3. Removes the existing DynamicAction types.

Fixes #412. Fixes #278. Fixes #68.

Remaining items

• fix tests
• fix doc tests
• check examples

[alice-i-cecile]

Closing this and doing this in a more incremental way.