SuccessorML extended text constants

[shwestrick]

Fixes #49 and additional progress on #47.

Command-line option -allow-extended-text-consts true allows for UTF-8 bytes within strings, for example:

val a = "🍰"
val x = print "🂡\n"

This can also be controlled from within an MLB following MLton conventions:

ann "allowExtendedTextConsts true" in
  foo.sml
end

Note that this feature does not check for validity of a UTF-8 byte sequence. We could add that later, if desired.