Start the Standard Library

[ahdinosaur]

Describe the solution you'd like

A core standard library based on: https://clojuredocs.org/quickref

At the moment, a function body can only be a Rimu expression. But for a standard library, we need to be able to access things in Rust-land, such as the length of a list's vec.

TODO Checklist

• Change Value's Function into an enum
  • Existing case is Function::Rimu
  • New case is Function::Rust, where the enum references a Rust function.
• Create rimu-stdlib crate
  • Create length function

Describe alternatives you've considered

Additional context