Cast expressions

[brendanzab]

Host expressions should have include a cast expression to allow for explicit casts between numeric types. These should be in the form:

host-expr ::=
    ...
    host-expr "as" host-type

Example:

struct {
    width: u32le,
    height: u8,
    data: [u8; width * height as u32],
}

• Parsing: Note that the challenge is that cast expressions require that we be able to also parse host types, rather than just binary types. We could start off hard-coding the base types to avoid issues regarding variable capture and environments.
• Type checking: We'll have to check that the input and output types are numeric
• Lowering to IR: We'll need to add another variant to the Expr type
• Lowering to Rust: This should be relatively simple - just use as in Rust. Might be harder to maintain consistent semantics across other languages though. 🤔