buffer tells fiddle to create a buffer of a number of bytes.

skip_to tells fiddle to skip to some new offset. It's essentially an
unnamed buffer

This is to make anonymous expansion better and cleaner. It gets rid of
the hash-mark hack introduced earlier.

This is for expressions which must be calculatable at compile time.

This helps to catch bugs in the compiler, specifically ones related to
mixing up bits and bytes.

This remove the burden of generating names from the backend.

This makes deriving much easier and cleans up the messy contexts in
GenericTree and elsewhere at the cost of slightly more obtuse syntax.

Big change. Implements qualification, which separates the qualification
concerns from the ConsistencyCheck phase.

I'm getting close to implementing a backend.