Procmacro to derive unary/binary function definitions

[antiguru]

Similar to the sqlfunc! macro, this proc macro derives the helper traits for unary and binary functions.

Here's a binary function for which we generate the EagerBinaryFunc trait implementation:

#[sqlfunc(is_monotone = (true, true), output_type = i16, is_infix_op = true, sqlname="+")]
fn add_int16<'a>(a: Datum<'a>, b: Datum<'a>) -> Result<Datum<'a>, EvalError> {
    a.unwrap_int16()
        .checked_add(b.unwrap_int16())
        .ok_or(EvalError::NumericFieldOverflow)
        .map(Datum::from)
}

The parameters control the behavior of the code that we generate and allow the function to override the default behavior.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ParkMyCar]

Before merging could you take a pass on the comments? Especially would love to see the documentation improved but overall love this PR!