Regex Library in Rust

Features implemented:
- Basic character matching
- \d for digits
- \w for word characters (digits, letters, and _)
- \s for whitespace
- \D for not digits
- \W for not word characters
- \S for not whitespace

Modifiers
- + for 1 or more
- ? for 0 or 1
- * for 0 or more

Basic testing only, so the features implemented might not work as described.
Complexity is O(N + K) where N is the length of the string to search in and K is the length of the regex