Resolve build warnings

[davidrusu]

On build, I see a lot of unused import warnings, e.g.

warning: unused import: `utils::read_base_field`
  --> taiga_halo2/src/circuit/vp_examples.rs:12:5
   |
12 |     utils::read_base_field,
   |     ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

This PR simply pushes those imports into the Borsh serializer impls where they are used

(chains off of #259)

[XuyangSong]

Thank you for your contribution. @davidrusu