[WIP] Add arithmetic sub-expressions to Ion's syntax

Created by: huntergoldstein

Fixes: Closes #243 (closed)

Changes introduced by this pull request:

• Builtin calc functionality is now exposed to the rest of the crate
• Added support for the $((...)) syntax form which is equivalent to calc '...' with some syntactic sugar for referring to variables

Drawbacks:

• The calc builtin is now tightly coupled to the rest of the crate: I don't feel this is desirable

TODOs:

• Add tests for parsing at statement and word level
• Revise flags and logic in parser::statements; this could be cleaner IMO

State: Core functionality should be there, needs some polish

Very nice implementation that you have for arithmetic. Whenever you are ready, we can merge this work. I would like to eventually split the logic up in parser::statements into diverging nano-parsers. Current implementation is largely the work of duct-taping it repeatedly over time as issues are discovered.

Created by: huntergoldstein

Ready to merge!