fix: added lowering mechanism to support functions with the reference to * return type

[Angus-Bethke-Bachmann]

Added:

• A new "reference to return" lowerer that will lower functions with a return type of "reference to" into code that can be parsed by codegen

Changed:

• Allowing functions with a "reference to" return type
• Updated validations for existing invalid "reference to" assignment to support clearer error messages now that this is allowed by the lexer
• Augmented codegen to support passing parameters by reference if both sides are "reference to"

Testing:

• Added codegen unit test to verify the generated code for the lowered code
• Added lit tests to verify the correct outcome for functions with a return type of "reference to"