+ # Mangle a string to be a unique string only made of alphanumeric characters and underscores.
+ #
+ # This method is injective (two different inputs never produce the same
+ # output) and the returned string always respect the following rules:
+ #
+ # * Contains only US-ASCII letters, digits and underscores.
+ # * Never starts with a digit.
+ # * Never contains two contiguous underscores.
+ #
+ # assert "42_is/The answer!".to_cmangle == "_52d2_is_47dThe_32danswer_33d"
+ # assert "__d".to_cmangle == "_95d_d"
+ # assert "_42".to_cmangle == "_95d42"
+ # assert "foo".to_cmangle == "foo"
+ # assert "".to_cmangle == ""