Address remaining mismatch with openqasm3 AST type hints

[PhilReinhold]

#48 Surfaced 2 remaining issues where the AST we produce is not consistent with the openqasm3 AST type hints.

1. Pragmas are not statements, I think to keep them from being added anywhere other than at the top level. I would guess they should be added to the set of valid types in a Program.statements field.
2. Box statements only allow QuantumStatement which does not include e.g. play openpulse function calls.