Using chars instead of strings where possible and &str instead of
&String where sufficient.

? with Ok() is needless where just returning the result is fine.
Remove needless references

- &[T] (slice) is more flexible than &Vec<T> - it can accept arrays,
slices, and Vec references
- &Vec<T> forces callers to have a Vec, while &[T] works with any
contiguous collection
- There's no performance difference since &Vec<T> automatically derefs
to &[T] when needed

If expressions can leave unaccounted options, whereas match is stricter.

…umber range finding

splitn() is needless in this case

replace x == None with .is_none()