Deal with warnings in zkey.rs

[oskarth]

Problem

Currently zkey.rs generates a bunch of warnings. Since rust clippy is run as part of the linting CI process (https://github.com/gakonst/ark-circom/blob/master/.github/workflows/ci.yml#L61), these should probably be dealt with.

Details

> cargo clippy
    Checking ark-circom v0.1.0 (/home/oskarth/git/ark-circom)
warning: field is never read: `size`
  --> src/zkey.rs:46:5
   |
46 |     size: usize,
   |     ^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: field is never read: `ftype`
  --> src/zkey.rs:61:5
   |
61 |     ftype: String,
   |     ^^^^^^^^^^^^^

warning: field is never read: `version`
  --> src/zkey.rs:62:5
   |
62 |     version: u32,
   |     ^^^^^^^^^^^^

warning: field is never read: `n8q`
   --> src/zkey.rs:258:5
    |
258 |     n8q: u32,
    |     ^^^^^^^^

warning: field is never read: `q`
   --> src/zkey.rs:259:5
    |
259 |     q: BigInteger256,
    |     ^^^^^^^^^^^^^^^^

warning: field is never read: `n8r`
   --> src/zkey.rs:261:5
    |
261 |     n8r: u32,
    |     ^^^^^^^^

warning: field is never read: `r`
   --> src/zkey.rs:262:5
    |
262 |     r: BigInteger256,
    |     ^^^^^^^^^^^^^^^^

warning: field is never read: `power`
   --> src/zkey.rs:268:5
    |
268 |     power: u32,
    |     ^^^^^^^^^^

warning: `ark-circom` (lib) generated 8 warnings