Adds arithmetic operators as separate nodes & fixes casts

[johnedquinn]

Description

• Adds arithmetic operators as separate nodes. This allows for us to type the arithmetic operators how we please. I followed SQL Server's rules for typing.
• Fixes casts to invalid types for conformance. The conformance tests allow them to pass -- so I made them warnings instead of errors.
• Fixes function resolution (static) for dynamic arguments.
  • Previously, 1 > some_dynamic_value would result in Fn.Gt(INT, INT) -> BOOL. While this had the most number of static exact matches, it didn't necessarily have the most number of runtime exact matches -- causing behavior that would break the gradual guarantee. Fixed this by checking whether any argument is dynamic.
• Simplifies dynamic dispatch (runtime) by following the same rules as function resolution.

Other Information

• Updated Unreleased Section in CHANGELOG: NO
• Any backward-incompatible changes? YES
  • Added nodes specifically for the arithmetic operators.
• Any new external dependencies? NO
• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? YES

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[johnedquinn]

Closing in favor of #1533.