build: make rust 1.78.0-nightly happy

[SadPencil]

I installed the latest rust nightly build, 1.78.0-nightly, and got compilation errors running cargo build in mpc-snarks directory:

error: the item `Vec` is imported redundantly
 --> /home/user/zkp-mpc-pianist/r1cs-std/src/bits/mod.rs:3:5
  |
3 |     Vec,
  |     ^^^
 --> /rustc/2d24fe591f30386d6d5fc2bb941c78d7266bf10f/library/std/src/prelude/mod.rs:115:13
  |
  = note: the item `Vec` is already defined here
  |
note: the lint level is defined here
 --> /home/user/zkp-mpc-pianist/r1cs-std/src/lib.rs:6:5
  |
6 |     unused,
  |     ^^^^^^
  = note: `#[deny(unused_imports)]` implied by `#[deny(unused)]`

There are hundreds of errors like this.

I've fixed all compilation errors in this PR. Reference: rust-lang/rust#121708 (comment)

Note that this PR requires #5 as a prerequisite.