Don't silently fail with unsafe strings

[hiemanshu]

• I'm submitting a ...
  [x] bug report
  [ ] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

• Summary

If a prefix or key includes "unsafe" characters like : or | the initialization / memoize-ation can silently fail. Ideally memoredis throws an error about unsafe strings rather than silently failing and not returning anything.

https://github.com/politics-rewired/memoredis/blob/main/src/lib/memoizer.ts#L78-L80

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. StackOverflow, personal fork, etc.)