[#199] owner: Rename to user, refactor and doc

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

user/doc.go

@@ -0,0 +1,55 @@
+/*
+Package user provides functionality related to NeoFS users.
+
+User identity is reflected in ID type. Each user has its own unique identifier
+within the same network.
+
+NeoFS user identification is compatible with Neo accounts:
+	import "github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	import "github.com/nspcc-dev/neo-go/pkg/crypto/hash"
+
+	var id user.ID
+
+	var scriptHash util.Uint160 // user account in NeoFS
+	id.SetScriptHash(scriptHash)
+
+	var key keys.PublicKey // user's public key
+	user.IDFromKey(&id, k.PrivateKey.PublicKey)
+
+ID is compatible with the NeoFS Smart Contract API:
+	var id user.ID
+	// ...
+
+	wallet := id.WalletBytes()
+
+	// use wallet in call
+
+Encoding/decoding mechanisms are used to transfer identifiers:
+	var id user.ID
+	// ...
+
+	s := id.EncodeToString() // on transmitter
+	err = id.DecodeString(s) // on receiver
+
+Instances can be also used to process NeoFS API 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.OwnerID
+	id.WriteToV2(&msg)
+
+	// send msg
+
+On server side:
+	// recv msg
+
+	var id user.ID
+
+	err := id.ReadFromV2(msg)
+	// ...
+
+	// process id
+*/
+package user