Avoid asking proxy generators to create Fakes that they can't

[blairconrad]

Currently we try to invoke the proxy generators, often once per available constructor, when making a Fake, even if the proxy generator can't possibly succeed.

For example, we try to use CastleDynamicProxyGenerator to create a string, even though strings are sealed (the CastleDynamicProxyGenerator exits early, but not before we create constructor arguments).

(From #1347.)

[blairconrad]

This change has been released as part of FakeItEasy 4.7.0.