Remove "unsafe" block in BLST bindings

[paulhauner]

Description

There is an unsafe block in our crypto/bls crate that I think should be moved into blst:

lighthouse/crypto/bls/src/impls/blst.rs

Lines 67 to 73 in 32338bc

	// TODO: remove this `unsafe` code-block once we get a safe option from `blst`.
	//
	// See https://github.com/supranational/blst/issues/13
	unsafe {
	blst::blst_scalar_from_uint64(rand_i.as_mut_ptr(), vals.as_ptr());
	rands.push(rand_i.assume_init());
	}

To resolve

Follow up on supranational/blst#13

[ayushmishra2005]

@paulhauner I would like to pick this issue. As a first step, I have raised a PR in blst library supranational/blst#52 to expose safe blst_scalar constructors.

Once this PR gets merged, I would make corresponding changes here

lighthouse/crypto/bls/src/impls/blst.rs

Lines 67 to 73 in 32338bc

	// TODO: remove this `unsafe` code-block once we get a safe option from `blst`.
	//
	// See https://github.com/supranational/blst/issues/13
	unsafe {
	blst::blst_scalar_from_uint64(rand_i.as_mut_ptr(), vals.as_ptr());
	rands.push(rand_i.assume_init());
	}

[paulhauner]

I'm going to close this since BLST doesn't seem interested in providing the functionality.