A couple of extensions to `with` that would be worthwhile

[Andrei Alexandrescu]

We have an awkward idiom in the dmd source code:

https://github.com/dlang/dmd/search?q=extendedPathThen
https://github.com/dlang/dmd/search?q=toWStringzThen

It's a tail wagging the proverbial dog. Clearly we could do with better 
ways. We can improve the `with` statement as follows:

1. Accept named `with`:

with (auto xpath = extendPath(name)) {
     // the only name injected here is xpath
}

All destructors for the expression involved in `with` are called AFTER 
the with is done.

2. Accept primitive types in named `with`:

with (auto xpath = extendPath(name).str) {
     // the only name injected here is xpath
     // assume str has type wstring, then xpath
     // also has type string
}

Again, the destructor of the temporary object created is called AFTER 
the with statement ends, even though the object is only used as a temporary.

3. Accept `with` with colon:

with (auto xpath = extendPath(name).str):

The meaning is like a scoped `with` that extends though the end of the 
scope.

4. Accept `with` at top level.

All `with` forms that take a type should be accepted at top level. This 
allows entire modules or fragments thereof to look names up in a 
flexible manner.