Implement some of good practices for the contracts

[AnnaShaleva]

Some of our contracts contain issues that need to be refactored:

1. Replace common.BytesEqual usages with built-in (a Hash160) Equals(b interface{}) bool where possible (see

   neofs-contract/alphabet/alphabet_contract.go

   Line 30 in dc89137

   if !common.BytesEqual(caller, []byte(gas.Hash)) && !common.BytesEqual(caller, []byte(neo.Hash)) {

   as an example). The same thing can be done for Hash256 comparison cases.
2. Use pre-defined constants where appropriate (interop.Hash160Len, interop.PublicKeyCompressedLen, etc.). There are still several cases where numbers are use instead (see

   neofs-contract/container/container_contract.go

   Line 472 in dc89137

   containerID := eACL[offset : offset+32]

   as an example).
3. Use the most specific parameter types for method signatures where appropriate (e.g. interop.Hash160 instead of []byte), see

   neofs-contract/neofs/neofs_contract.go

   Line 371 in dc89137

   func Bind(user []byte, keys []interop.PublicKey) {

   as an example.

Ref. #304.

[AnnaShaleva]

Suggestions 1 and 2 are implemented in #317. Suggestion 3 is left to be implemented in a separate PR, as it requires careful review of the contract API's users in the node's code.