Make IProxyGenerator (+ IFakeCallProcessorProvider and IFakeCallProcessor) internal

In #373 we introduced `IFakeCallProcessorProvider` and `IFakeCallProcessor`, which had to be public due to the usage in `IProxyGenerator`.

This PR proposes making `IProxyGenerator`, as a part of #374,  internal to allow making the introduced interfaces internal and hide `FakeManager.Process` (explicit implementation of `IFakeCallProcessor.Process`).

Note that `IProxyGenerator` is an infrastructure interface, which is just used in `FakeObjectCreator` and it was public since initial commit (without stating a reason).

Further note that we already introduced a breaking change in `IProxyGenerator` in #373 by adding the `fakeCallProcessorProvider` parameters, so we *could* use this chance before releasing the next version to make this interface internal.

@ contributors: What do you think?