added benchmarks for G_T

[alinush]

Added some missing benchmarks for $\mathbb{G}_T$ operations.

Note: The rust_toolchain file and the hardcoded version in Cargo.toml caused problems for me. However, I can revert those changes if need be.

[vmx]

I now also see an error locally on master. I'll have a look (it should be independent of this PR).

[vmx]

CI should be fixed now, could you please rebase your change?

[alinush]

Rebased and formatted with cargo fmt!