Add signature literals (#505)

* Add signature literals + handle system alias types

* Add tests for signature literals + encode/decode

* Add to CHANGELOG

* Add in documentation

* Additional documentation

docs/sophia_features.md

@@ -573,7 +573,7 @@ Sophia has the following types:
 | state                | `state{ owner = Call.origin, magic_key = #a298105f }`                                                                               |
 | event                | `EventX(0, "Hello")`                                                                                                                |
 | hash                 | `#000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f`                                                                 |
-| signature            | `#000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f` |
+| signature            | `sg_MhibzTP1wWzGCTjtPFr1TiPqRJrrJqw7auvEuF5i3FdoALWqXLBDY6xxRRNUSPHK3EQTnTzF12EyspkxrSMxVHKsZeSMj`, `#000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f000102030405060708090a0b0c0d0e0f` |
 | Chain.ttl            | `FixedTTL(1050)`, `RelativeTTL(50)`                                                                                                 |
 | oracle('a, 'b)       | `ok_2YNyxd6TRJPNrTcEDCe9ra59SVUdp9FR9qWC5msKZWYD9bP9z5`                                                                             |
 | oracle_query('a, 'b) | `oq_2oRvyowJuJnEkxy58Ckkw77XfWJrmRgmGaLzhdqb67SKEL1gPY`                                                                             |

docs/sophia_syntax.md

@@ -30,6 +30,7 @@ interface main using as for hiding
 - `ContractAddress` base58-encoded 32 byte contract address with `ct_` prefix
 - `OracleAddress` base58-encoded 32 byte oracle address with `ok_` prefix
 - `OracleQueryId` base58-encoded 32 byte oracle query id with `oq_` prefix
+- `Signature` base58-encoded 64 byte cryptographic signature with `sg_` prefix
 
 Valid string escape codes are
 
@@ -239,6 +240,7 @@ Expr ::= '(' LamArgs ')' '=>' Block(Stmt)   // Anonymous function    (x) => x +
        | Int | Bytes | String | Char        // Literals              123, 0xff, #00abc123, "foo", '%'
        | AccountAddress | ContractAddress   // Chain identifiers
        | OracleAddress | OracleQueryId      // Chain identifiers
+       | Signature                          // Signature
        | '???'                              // Hole expression       1 + ???
 
 Generator ::= Pattern '<-' Expr   // Generator