Make IProxyGenerator (+ IFakeCallProcessorProvider and IFakeCallProcessor) internal

[ulrichb]

(@adamralph: follow up change for #368)

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?

[ulrichb]

Note that we could also internalize ProxyGeneratorResult because this type is an infrastructure DTO (represents the output of IProxyGenerator) and was also forced to be public because of the public IProxyGenerator.

[blairconrad]

@ulrichb, your point about ProxyGeneratorResult is a good one, but since we didn't make anything worse in #373 (I know, we did change the class a little bit, but didn't make it depend on new interfaces or anything), I think we should not change ProxyGeneratorResult for this PR. It can be examined as part of #374, and I'll make a note in the Giant Table in that issue.

Otherwise, I think we fall into the trap of chasing down every visible FakeItEasy type that was in any way modified by #373, and I think that's too much work for this issue. The other changes #373 at least involved previously-existing (and visible) types.

[blairconrad]

To summarize, this change will internalize two new interfaces added in #373, as well as IProxyGenerator (which uses one of the new interfaces, and forced them to become public). IProxyGenerator is public, but is not useful to clients of FakeItEasy. Accordingly, I consider this to be a non-breaking change.

@adamralph, I'm happy with the change, and would include it in release 1.25.0, if you've 2 or 3 minutes (literally—it's very small) to give it your blessing.

[adamralph]

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