Improve Built-In types and literals

[JacquelineCasey]

• Write literal_fits
• Literals can definitely use some work overall
  • We need a way to write up to i64 and u64 literals, with validation and also proper conversion later on.
• Implement rule that main must return i32.
• Test unit type?
• Builtin types beyond unit and i32
  • Likely we will want
  • Requires an actual type analysis. I'd like to have one that associates a type to every expression using the identity in NodeData.
  • Requires a conversion AST node, ergo requires the ability to modify the AST.
  • We need to generate code for this AST node, which cares mostly about alignment.
  • We might need some support from the VM to handle narrowing conversions gracefully.
  • Or maybe all conversions gracefully. Byte order is a nuissance.