allow optional lists and maps

[sxlijin]

We currently allow neither of these properties to be declared:

class Foo {
  a string[]?
  b map<string, string>?
}

because the parser, in its current error-correcting behavior, coerces an unset parameter into an empty list / map.

However, we've had numerous asks for this, and there's no good reason to impose this (somewhat artificial) restriction on our users, so we should just allow this in the type system and parser.

Users to update:

• gabriel_syme