enforce interface-functions to be external-declared

- libsolidity: Enforce interface-functions must be external-declared.
- Changelog adapted to reflect changes wrt. functions in interfaces.
- test: Adjustments according to prior interface-function changes.
- tests: Adapting SolidityEndToEndTest to interface-function change.
- docs: Adapted documentation to interface-function change.

docs/contracts.rst

@@ -1150,6 +1150,7 @@ Interfaces
 Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions:
 
 - Cannot inherit other contracts or interfaces.
+- All declared functions must be external.
 - Cannot define constructor.
 - Cannot define variables.
 - Cannot define structs.
@@ -1167,7 +1168,7 @@ Interfaces are denoted by their own keyword:
     pragma solidity ^0.4.11;
 
     interface Token {
-        function transfer(address recipient, uint amount) public;
+        function transfer(address recipient, uint amount) external;
     }
 
 Contracts can inherit interfaces as they would inherit other contracts.

docs/security-considerations.rst

@@ -203,7 +203,7 @@ Now someone tricks you into sending ether to the address of this attack wallet:
     pragma solidity >0.4.24;
 
     interface TxUserWallet {
-        function transferTo(address dest, uint amount) public;
+        function transferTo(address dest, uint amount) external;
     }
 
     contract TxAttackWallet {