`rash` - A blazingly fast memory-safe POSIX shell

# Why?

I want to learn Rust and see how much effort I can put into making an interoperable software.

# Roadmap

- [x] Execute commands
- [ ] Variables
- [ ] Environment variables
- [ ] Builtins
  - [ ] `cd`
  - [ ] `set`
  - [ ] `alias`
  - [ ] `type`
- [ ] Parse commands
- [ ] Command-line flags
- [ ] PS1