Rename and refactor for XamlX structure and layering.

[jkoritzinsky]

Rename XamlIL -> XamlX. Refactor out base classes for the compilers for 3 built-in compiler types, declarative, imperative, and IL (each builds on previous). Make the backend generic on the "raw" emitter and the result type returned by the node emitters.

Keep support for implementing emit for specific backends on the node types themselves while also supporting using emitter objects exclusively.

Contributes to #8.

Targets the history-rewritten xamlx-rename branch. Superscedes #10

cc: @kekekeks

[kekekeks]

Why are we removing the 'X' postfix? Originally it was intended to differentiate between XAML engines. I'd rather avoid name clashes between IXamlXType and IXamlType. It's compiler internal anyway, so we don't have to worry about the XAML engine name leaking to the user-facing API.

It also seems to be a plain find/replace since it affects even xmlns urls and benchmark names.

[jkoritzinsky]

My thought was that we already have namespaces differentiating the types so we don’t need to also differentiate the type in the name.

[kekekeks]

Another git filter-branch it is then. I'll do a script to rename each type individually, I think.

[kekekeks]

Branch with updated history rewrite is here - https://github.com/kekekeks/XamlIl/tree/xamlx-rename2

I don't think we need to rename anything else.

You can find your current PR rebased on top of that branch in this commit (cherry-pick it or something).

[kekekeks]

Also, the current PR doesn't build because ILEmitContext doesn't implement LoadLocalValue.

[kekekeks]

Also, != default no longer works with generic type parameters in C# 9.