Sanitize stamps

[LiefEsbenshade]

Stamps can currently be any vector that can be coerced to a character string via paste, but special characters can cause issues in the file system (e.g. /).

Sanitizing steps - are these right?:

1. Should take all not alpha-numeric characters and strip them out (replace with _ ?).
   • or just strip out special characters that mess with the file system?
2. Add an error if provided stamp is not a vector of length 1?
3. Limit length of stamps?