[Tech debt] Cleanup repo

[mratsim]

Part of the tech debt week status-im/nimbus-eth2#867

With the varying schemes we had we have accumulated some cruft in the repo.
In preparation for auditing, the code paths that are not used should be cleaned up.

For example

• bls_old_spec: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/bls_old_spec.nim which was kept in case we needed to revert to 0.9.x BLS signature scheme
• hashToG2: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/common.nim#L799
• random for secret key generation: https://github.com/status-im/nim-blscurve/blob/9efb20df/blscurve/common.nim#L861

[mratsim]

Closed by #49

[mratsim]

Reopening

Another cleanup round:

nim-blscurve/blscurve/common.nim

Lines 760 to 801 in b435f1a

	proc hashToG2*(msgctx: sha256, domain: Domain): ECP2_BLS381 =
	## Perform transformation of sha2-256 context (which must be already
	## updated with ``message``) over domain ``domain`` to point in G2.
	## https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/bls_signature.md#hash_to_g2
	var
	xre, xim: BIG_384
	x, one: FP2_BLS381
	var ctx1 = msgctx
	ctx1.update(domain)
	var ctx2 = ctx1
	# Update hash context with coordinate marker
	ctx1.update([0x01'u8])
	ctx2.update([0x02'u8])
	# Obtain result hash
	var xrehash = ctx1.finish()
	var ximhash = ctx2.finish()
	discard xre.fromBytes(xrehash.data)
	discard xim.fromBytes(ximhash.data)
	# Convert (xre, xim) to FP2.
	x.fromBigs(xre, xim)
	# Set FP2 One
	one.setOne()
	while true:
	# Without normalization of `x` 32bit version will fail
	norm(x)
	if ECP2_BLS381_setx(addr result, addr x) == 1:
	break
	# Increment `x` by FP2(1, 0)
	x = add(x, one)
	# Get biggest Y
	let negy = neg(result.y)
	# Reverting sign of ``y`` only when ``y`` less then negated one.
	if cmp(result.y, negy) < 0:
	result.y = negy
	# Scale with G2 CoFactor
	result = mulCoFactor(result)

was missed.

And when HashToCurve v7 lands and the v5 can be completely retired (probably soon as Schlesi testnet is down), we need to cleanup v5 code.

[mratsim]

Closed by #65