Redo the global (state) structure of the translator.

Redo the global (state) structure of the translator.

This gives the VHDL module its own state and moves the Entity for each
function into that state. The AST.EntityDec and AST.ArchBody are no longer
stored in the state, but simply returned directly.

The State class used is changed from the one from the mtl library to the
one from the transformers library, since that one integrates nicely with
the data-accessors library. This integration (together with the
simplification of the states) pretty much removes the need for all
manually defined accessor function.

This change breaks support for builtin functions (hwxor, hwnot, etc.),
which will be fixed in a subsequent commit. Also, custom types are not
longer output right now, but there is infrastructure in place to do better
type collection.