Move ERC1820 and ERC777 out of drafts (#1742)

* Moved ERC1820 related contracts out of drafts and into introspection.

* Moved ERC777 related contracts out of drafts and into token.

(cherry picked from commit c794c96617)

contracts/mocks/ERC1820ImplementerMock.sol

@@ -1,6 +1,6 @@
 pragma solidity ^0.5.0;
 
-import "../drafts/ERC1820Implementer.sol";
+import "../introspection/ERC1820Implementer.sol";
 
 contract ERC1820ImplementerMock is ERC1820Implementer {
     function registerInterfaceForAddress(bytes32 interfaceHash, address account) public {

contracts/mocks/ERC777Mock.sol

@@ -1,6 +1,6 @@
 pragma solidity ^0.5.0;
 
-import "../drafts/ERC777/ERC777.sol";
+import "../token/ERC777/ERC777.sol";
 
 contract ERC777Mock is ERC777 {
     constructor(

contracts/mocks/ERC777SenderRecipientMock.sol

@@ -1,10 +1,10 @@
 pragma solidity ^0.5.0;
 
-import "../drafts/ERC777/IERC777.sol";
-import "../drafts/ERC777/IERC777Sender.sol";
-import "../drafts/ERC777/IERC777Recipient.sol";
-import "../drafts/IERC1820Registry.sol";
-import "../drafts/ERC1820Implementer.sol";
+import "../token/ERC777/IERC777.sol";
+import "../token/ERC777/IERC777Sender.sol";
+import "../token/ERC777/IERC777Recipient.sol";
+import "../introspection/IERC1820Registry.sol";
+import "../introspection/ERC1820Implementer.sol";
 
 contract ERC777SenderRecipientMock is IERC777Sender, IERC777Recipient, ERC1820Implementer {
     event TokensToSendCalled(

contracts/token/ERC777/ERC777.sol

@@ -6,7 +6,7 @@ import "./IERC777Sender.sol";
 import "../../token/ERC20/IERC20.sol";
 import "../../math/SafeMath.sol";
 import "../../utils/Address.sol";
-import "../IERC1820Registry.sol";
+import "../../introspection/IERC1820Registry.sol";
 
 /**
  * @title ERC777 token implementation, with granularity harcoded to 1.