Rename builtin natural to nat and boolean to bool

[paulcadman]

The builtin labels for natural and boolean are renamed to nat and bool.

This PR also updates the reference to the stdlib containing updates to the stdlib Nat and Bool definitions.

builtin nat
inductive Nat  {
  zero : Nat;
  suc : Nat → Nat;
};

builtin bool
inductive Bool {
  true : Bool;
  false : Bool;
};

Now that Bool is builtin, we do not need the BackendBool type and bridge from the Juvix Bool inductive type and the bool C type.

Updating stdlib submodule

After merging this PR - the juvix-stdlib PR anoma/juvix-stdlib#18 can be merged, then the juvix-stdlib submodule should be updated to point to juvix-stdlib main.