[#190] Refactor cryptographic functionality

Remove `signature` and `util/signature` packages. Re-implement their
functionality in new `crypto` package. Generalize the approach of
digital signature computation and verification by adding `Signer` and
`PublicKey` primitives similar to standard `crypto` package. Support
already exising in protocol signature schemes.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

crypto/doc.go

@@ -0,0 +1,48 @@
+/*
+Package neofscrypto collects NeoFS cryptographic primitives.
+
+Signer type unifies entities for signing NeoFS data.
+	// instantiate Signer
+	// select data to be signed
+
+	var sig Signature
+
+	err := sig.Calculate(signer, data)
+	// ...
+
+	// attach signature to the request
+
+SDK natively supports several signature schemes that are implemented
+in nested packages.
+
+PublicKey allows to verify signatures.
+	// get signature to be verified
+	// compose signed data
+
+	isValid := sig.Verify(data)
+	// ...
+
+Signature can be also used to process NeoFS API V2 protocol messages
+(see neo.fs.v2.refs package in https://github.com/nspcc-dev/neofs-api).
+
+On client side:
+	import "github.com/nspcc-dev/neofs-api-go/v2/refs"
+
+	var msg refs.Signature
+	sig.WriteToV2(&msg)
+
+	// send msg
+
+On server side:
+	// recv msg
+
+	var sig neofscrypto.Signature
+	sig.ReadFromV2(msg)
+
+	// process sig
+
+Using package types in an application is recommended to potentially work with
+different protocol versions with which these types are compatible.
+
+*/
+package neofscrypto