IsInstanceOf refactoring

__Big (non breaking) Change__:
. Migrated IsInstanceOf<T> (and IsNotInstanceOf) to FluentCheck and FluentStructCheck. It means they are no longer *implemented through extension methods*. The upside is that they are available for **ALL TYPES** now, the downside is that you cannot specialize their implementation for a specific type.

__Other improvements__:
. Removed now useless exension methods for IsInstanceOf
. Added tests to complete coverage
. Added a new test fixture dedicated to exploratory tests: EdgeCasesTest.cs
. Added tests for IsNotInstanceOf for nullable types