SecretKey -> PublicKey preconditions docs + fixes.

[mratsim]

This fixes #114 and status-im/nimbus-eth2#2695

Overview

In BLSCurve, preconditions checks for:

• SecretKey: 0 < SK < r (BLS12-381 curve order)
• PublicKey: PK != infinity and PK in G1 subgroup

are done at deserialization time.

nim-blscurve/blscurve/blst/bls_sig_io.nim

Lines 149 to 168 in 50dbaba

	func fromBytes*(
	obj: var SecretKey,
	raw: openarray[byte] or array[32, byte]
	): bool {.inline.} =
	## Initialize a BLS secret key from
	## its raw bytes representation.
	## Returns true on success and false otherwise
	const L = 32
	when raw is array:
	obj.scalar.blst_scalar_from_bendian(raw)
	else:
	if raw.len != 32:
	return false
	let pa = cast[ptr array[L, byte]](raw[0].unsafeAddr)
	obj.scalar.blst_scalar_from_bendian(pa[])
	if obj.vec_is_zero():
	return false
	if not obj.scalar.blst_sk_check().bool:
	return false
	return true

nim-blscurve/blscurve/blst/bls_sig_io.nim

Lines 81 to 99 in 50dbaba

	func fromBytes*(
	obj: var PublicKey,
	raw: array[48, byte] or array[96, byte]
	): bool {.inline.} =
	## Initialize a BLS signature scheme object from
	## its raw bytes representation.
	## Returns true on success and false otherwise
	result =
	when raw.len == 48:
	obj.point.blst_p1_uncompress(raw) == BLST_SUCCESS
	elif raw.len == 96:
	obj.point.blst_p1_deserialize(raw) == BLST_SUCCESS
	else: false
	# Infinity public keys are not allowed
	if result:
	result = not bool obj.point.blst_p1_affine_is_inf()
	if result:
	result = bool obj.point.blst_p1_affine_in_g1()

This means that procedures that rely on those types, like publicFromSecret can assume that those preconditions are already respected.
This has been clarified in a dedicated document https://github.com/status-im/nim-blscurve/blob/9b7366cdc7df14c87930c3ed0243ab551a5daf56/docs/bls_types_guarantees.md

Tests have been added to ensure that #114 was just a documentation issue and revealed that the MIRACL backend (not used in production) could actually deserialize an invalid secret key > r.

Furthermore, for defensive programming reason, an extra precondition check in publicFromSecret has been added to recheck that the input respects 0 < SK < r. That check should never fail.

[arnetheduck]

we can probably slap a requiresInit on it to catch any zero-initialized stragglers