parser: rename Tree to Ast and move all Ast classes into c99.parser.ast

What we are building at this stage really is an AST, so the class names
should better reflect that fact. Further, this is just temporary and
will go away, so we are moving all these temporary "AST" classes into
a separate package to minimize the pollution. And we are preserving the
word "tree" for when we start building the actual useful "decorated trees".

Recently the term AST has been used to mean "decorated syntax tree". That
is fine (we are not the abbreviation police), and we would be fine doing it
too, if we hadn't happened to build real "abstract syntax trees" first.
So, now we are forced to use the correct terms.