fix: fix rust warning for unused import

[stringhandler]

Adds in this allow for this warning in rust-simplicity.

warning: unused import: `crate::decode_bits`
 --> src\jet\init\elements.rs:6:5
  |
6 | use crate::decode_bits;
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

The import is used in the bitcoin jet, so it can't be excluded from the generation. My Haskell is not up to scratch enough for me to put in more complicated logic. That said, if I work out how to do it in future, I will put in another PR.