Generalize parsing of several @tags in DocParser.

Remove bunch of duplication by parsing similar tags with a more
generic functions.

Also avoid repeating regexes by not testing for the exact tagname
any more after the first initial check.  The new helper method
parse_tag_as() replaces in lots of places two lines of code with
just one.
Replace big IF in DocParser with hash lookup.

The code should be now both cleaner and faster (we're no more calling
look(/regex/) repeatedly to decide which at_tag method to call).
Eliminate #parse_tag_as method of DocParser.

Just using the add_tag method like previously, but the actual
parsing of the @tag is performed in parse_at_tag method before
control flow reaches the method for parsing the concrete tag.
Reorganize and document DocFormatter.
Eliminate repeated usage of #skip_white.

Instead call it at the top level method, ensuring it will be
called always after parsing any tag.
Use #skip_horiz_white inside parsing @return.

The use of #skip_white must have been a mistake.
Prototype of @cfg tag implemented as a class.
Implement all member tags as builtin tag classes.
Implement all boolean @tags as builtin tag classes.
Implement all class-tags as builtin tag classes.
Implement class-list-tags as builtin tag classes.
Implement @param & @throws as builtin tag classes.
Implement @return as builtin tag class.
Simplify @type parsing method.
Implement @enum & @override as builtin tag classes.
Implement @type as builtin tag class.
Implement @xtype/ptype/ftype as builtin tag classes.
Refactor @inheritdoc implementation.

Extract parts of the parsing routine into separate methods so
that the #at_inheritdoc method works at a more higher level.
Implement @alias & @inheritdoc as builtin tag classes.
Remove old BUILTIN_TAGS logic within DocParser.

All builtin tags are now implemented as tag classes.