Support introspectable=no attribute, add warnings framework

This work allows us to move closer to replacing gtk-doc, among other
things.  We add a generic attribute "introspectable", and inside the
typelib compiler if we see "introspectable=no", we don't put it in the
typelib.  This replaces the hackish pre-filter for varargs with a much
more generic mechanism.

The varargs is now handled in the scanner, and we emit
introspectable=no for them.

Add generic metadata to Node with references to file/line/column,
which currently comes from symbols.

Add scanner options --warn-all and --warn-error.

https://bugzilla.gnome.org/show_bug.cgi?id=621570