Sort out my `string` library and Marshall's `strings` library

[codereport]

• Mine: https://github.com/codereport/bqn-code/blob/main/lib/string.bqn
• Marshall's: https://github.com/mlochbaum/bqn-libs/blob/b8b49451ef1dd15ec14dfda074a9e108fb68a671/strings.bqn

This involves resolving Join vs _join, Split vs Split and both ReplaceAlls