Incant

- Properly set the return type annotation when preparing a function.
- A hook override can now force a dependency to be promoted to a parameter (instead of being satisfied) by setting `Hook.factory` to `None`.
- Parameters with defaults are now supported for `incanter.prepare` and `incanter.a/invoke`.
- `incanter.a/incant` no longer uses `invoke` under the hood, to allow greater customization. Previous behavior can be replicated by `incant(prepare(fn))`.
- Optional arguments of dependencies can now be propagated to final function arguments. Keyword-only arguments of dependencies are still filtered out.

- Introduce `incanter.prepare`, and make `incanter.a/invoke` use it. `prepare` just generates the prepared injection wrapper for a function and returns it, without executing it.
- Remove `incanter.parameters`, since it's now equivalent to `inspect.signature(incanter.prepare(fn)).parameters`.
- Add the ability to pass hook overrides to `incanter.prepare`, and introduce the `incanter.Hook` class to make it more usable.

- Initial release.