allow dotted path notation everywhere where assignments are allowed
finally implement support for assignments in the toplevel
make Alternative a child of Repeat if it is found, so it is also regarded as one expression when assigning it
add Source.until_parser_end() to read tokens until the parser quits

(until the depth of the state drops, a parser could still get replaced)
dont nest iterators returned by consume, so consume need no arguments and read_scheme and read_markup neither