Dropped ITypedEntity casting for internal usage.

Library was using ITypedEntity for internal type checks/assertions. This
was correct from the architecture point of view, but created a lot of
additional calls which hit the performance. This is dropped in favour of
in-memory store calls outside entity proxies.

This also corrects some minor issues and adds support for file based
in-memory stores.
Merge remote-tracking branch 'origin/master'

Conflicts:
	RomanticWeb/EntityContext.cs
	Tests/RomanticWeb.Tests/EntityContextTests.cs