Bootstrap ruby20.y.
ruby18.y, ruby19.y: ruby20.y: formatting.
ruby20.y: use dot_or_colon for block_command.
ruby20.y: extract command: operation to fcall to match parse.y.
ruby20.y, lexer.rl: end interpolated string with tSTRING_DEND.
- ruby18.y, ruby19.y: BEGIN{} does not introduce a scope.
+ ruby20.y: better error message for BEGIN{} in a method body.
+ ruby20.y: "p begin 1.times do 1 end end".
+ ruby20.y: "p () {}".
+ ruby20.y: "f {|;\nvar\n|}".