Create Env type for environment
Initial execution implementation
Bind variables and function definitions
Add ReturnCode type
Start shell expansions
Execute commands
Implement builtins
Move cond commands to Bash.Config.Cond
ReturnCode -> ExitStatus
Move and implement `append`
Add dummy implementations of expansions
Move Value within Bash.Config.Types
Unset parameters when dirty
Move Env to Bash.Config.Env
Bash.Config.Types -> Bash.Config.Command
Isolate Script type
Remove lens library
Store functions as Scripts
Move types back to Bash.Config.Types
Separate function bodies and names