Improve field comparison error message

[jfecher]

Related issue(s)

Related to #498

Description

The type system used to allow field comparisons, but since these operations are invalid we would eventually fail in SSA with a compiler crash. This fixes the check in the type system to disallow comparison for fields, but allow eq and neq operations.

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

[jfecher]

Before:

thread 'main' panicked at 'not implemented: Field comparison is not implemented yet, try to cast arguments to integer type', crates/noirc_evaluator/src/ssa/acir_gen.rs:339:29
stack backtrace:
   0: rust_begin_unwind
             at /rustc/e7119a0300b87a3d670408ee8e847c6821b3ae80/library/std/src/panicking.rs:556:5
   1: core::panicking::panic_fmt
             at /rustc/e7119a0300b87a3d670408ee8e847c6821b3ae80/library/core/src/panicking.rs:142:14
   [... omitted ...]
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
exit 101

After:

error: Fields cannot be compared, try casting to an integer first
  ┌─ /Users/jakefecher/Code/Noir/noir/lessthan/src/main.nr:2:15
  │
2 │     constrain x < y;
  │               -----

error: aborting due to 1 previous errors
exit 1

[guipublic]

I think we should enable field comparison, but until then it is better to report the error. We can easily enable it in the type checker when we will do field comparison.