Rename and refactor package responsible for building AST

Rename from semantic to astbuilding.
Add declarations and environment classes

Add declaration class hierarchies for: objects, tags, labels and
nesc entities.

Environment represents a single scope of objects and tags. Labels will
be handled in different way in the future.

Update references to declarations and environments in AST nodes (done
partially). Fix imports in generated classes.
Build symbol table for objects

The most common cases are supported. There are some case to implement:
old-style parameters, function parameter declaration without name.

File paths are normalized, i.e. redundant occurrences of../, ./, etc
are removed when possible.

Each environment contains list of nested environments.
Add getAll() method in environment to enable proposals computation.