Switch to LINQ expressions

[naasking]

The current implementation is inherently limited to a small subset of C# terms, I use dynamic type tests to support IEnumerable-castable types, and I can't support any more complicated composite structures. If I switch to using LINQ expressions, I can support unification on all C# terms supported by LINQ, which is considerable greater.

For instance, I can support logic variables that initialize object fields or any kind of collection.