Reference contracts naming scheme

[anton-trunov]

As of now the reference-contracts directory looks like this:

.
├── FungibleToken-Mintable.scilla
├── FungibleToken-Operator.scilla
├── FungibleToken.scilla
├── MetaFungibleToken.scilla
├── multisig_wallet.scilla
├── nonfungible-token.scilla
├── style-guide.md
└── zrc6.scilla

and it's not clear what, for instance, nonfungible-token.scilla's ZRC number is.

One possible proposal could be to prepend all the contract names with their ZRC index like so:

.
├── zrc2_FungibleToken-Mintable.scilla
├── zrc2_FungibleToken-Operator.scilla
├── zrc2_FungibleToken.scilla
├── zrc3_MetaFungibleToken.scilla
├── zrc4_Multisig-Wallet.scilla
├── zrc1_NonFungibleToken.scilla
├── style-guide.md
└── zrc6_NonFungibleToken.scilla