🏗️ handle types in R; expect only `RealSexp` in Rust

[jdhoffa]

Currently, there are two effectively identical implementations of sum_with_threads to account for integer or float inputs.
We might reduce complexity by ensuring input types are floats within the R session itself, and then in Rust only handle the NumericTypedSexp::Real variant.

This generally seems to be the more idiomatic approach as per savvy documentation:
https://yutannihilation.github.io/savvy/guide/key_ideas.html#no-implicit-conversions

Relates to #25
Maybe supersedes #30

[jibarozzo]

Excellent! I didn't really think in #25 to handle it in the R session first. haha. This is a great approach.