We now also look at the available declarations from included header
files when autocompleting names.
Additionally, you can now request autocomplete on an empty token, which
brings up all available names, starting from the inner-most scope.
This parser will be used by the C++ langauge server to provide better
auto-complete (& maybe also other things in the future).
It is designed to be error tolerant, and keeps track of the position
spans of the AST nodes, which should be useful later for incremental
parsing.
