Fix warnings for BindingFactor, BindingFactorList and SigningShare functions

[natalieesk]

We're getting the following warnings. Either we should remove the unused functions or add tests for them.

warning: associated function `deserialize` is never used
   --> src/lib.rs:189:12
    |
184 | impl<C> BindingFactor<C>
    | ------------------------ associated function in this implementation
...
189 |     pub fn deserialize(
    |            ^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

warning: method `iter` is never used
   --> src/lib.rs:231:12
    |
220 | impl<C> BindingFactorList<C>
    | ---------------------------- method in this implementation
...
231 |     pub fn iter(&self) -> impl Iterator<Item = (&Identifier<C>, &BindingFactor<C>)> {
    |            ^^^^

warning: associated function `from_coefficients` is never used
   --> src/keys.rs:120:19
    |
88  | impl<C> SigningShare<C>
    | ----------------------- associated function in this implementation
...
120 |     pub(crate) fn from_coefficients(coefficients: &[Scalar<C>], peer: Identifier<C>) -> Self {
    |  

• BindingFactor::deserialize()
• BindingFactorList::iter()
• SigningShare::from_coefficients()