ast: inline a use of r5js.Datum#getType.
test: r5js.test.matchers.ScansAs_: take datum ctor directly, not r5js.Type.
parse: add r5js.parse.bnf.{list,vector} and use in r5js.Parser where possible.
parse: add r5js.parse.bnf.dottedList and use in r5js.Parser where possible.
parse: replace a use of r5js.parse.bnf.list with dottedList.