Extracted many extension classes from FakeExtensions. For #206.

Fixes #206.

I started extracting to classes by the type acted-upon, but saw some methods had already been put in method-specific files, so end up doing that, mostly.
I'm happy to change to a different scheme.

Except for relocating it, I mostly left the code untouched, except for hooking up FluentAssertions and adding `FakeItEasy.Tests.TestHelpers.ObjectAssertionsExtensions` to fluently check exceptions. It's not exactly as we had discussed in the Jabbr room so many months ago, but I'm reasonably pleased with it. Of course, if you see room for improvement, chime in. You can see it used in `DefaultExceptionThrowerTests.cs` and `WithAnyArgumentsExtensionsTests.cs`.