Some cleanup
- Add constructors to the Shell
- Remove ascii helpers, they weren't used and the std provides more complete functions
- Move functions to reduce the number of pub which aren't supposed to be public
- Move necessary functions (signal handler, sigttou trap) from the builder to the shell constructor
- Only one function was left in the builder, so move set_unique_pid to Shell (doesn't matter if this is called after the shell starts running I think)
- Move functions to impl blocks
- Ignore SIGTTOU in background jobs too
- Combine repetitive calls in functions
- Remove version function from library, it only make sense for the terminal, not the library