parse nullable types

[degory]

  x: int?;
  y: String?;

Ultimately want nullable-ness to be opt-in across all value and reference types and all accesses through nullables to require a preceding successful has-value operation (?), provable via data-flow analysis.
Can't implement this until we have a real middle-end - even foregoing the data-flow analysis, we cannot determine what legacy L to emit without symbol type information, as required behaviour would differ for value types vs reference types (value types need wrapping in a struct, with a has-value flag, reference types can stay as is, with null representing has-no-value)
Not sure in the interim that there's much point in bothering to parse this?

[degory]

Nice to have but can wait until after bootstrap