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.

test/libsolidity/syntaxTests/constructor/interface_constructor_new.sol

@@ -2,6 +2,6 @@ interface I {
 	constructor() public;
 }
 // ----
-// Warning: (15-36): Functions in interfaces should be declared external.
+// TypeError: (15-36): Functions in interfaces must be declared external.
 // TypeError: (15-36): Constructor cannot be defined in interfaces.
 // TypeError: (15-36): Constructor must be implemented if declared.

test/libsolidity/syntaxTests/constructor/interface_constructor_old.sol

@@ -3,6 +3,6 @@ interface I {
 }
 // ----
 // Warning: (15-35): Defining constructors as functions with the same name as the contract is deprecated. Use "constructor(...) { ... }" instead.
-// Warning: (15-35): Functions in interfaces should be declared external.
+// TypeError: (15-35): Functions in interfaces must be declared external.
 // TypeError: (15-35): Constructor cannot be defined in interfaces.
 // TypeError: (15-35): Constructor must be implemented if declared.

test/libsolidity/syntaxTests/nameAndTypeResolution/416_interface_function_bodies.sol

@@ -1,7 +1,6 @@
 interface I {
-    function f() public {
+    function f() external pure {
     }
 }
 // ----
-// TypeError: (18-45): Functions in interfaces cannot have an implementation.
-// Warning: (18-45): Functions in interfaces should be declared external.
+// TypeError: (18-52): Functions in interfaces cannot have an implementation.

test/libsolidity/syntaxTests/nameAndTypeResolution/421_interface_function_parameters.sol

@@ -1,5 +1,4 @@
 interface I {
-    function f(uint a) public returns (bool);
+    function f(uint a) external returns (bool);
 }
 // ----
-// Warning: (18-59): Functions in interfaces should be declared external.

test/libsolidity/syntaxTests/parsing/interface_basic.sol

@@ -1,5 +1,4 @@
 interface Interface {
-	function f() public;
+	function f() external;
 }
 // ----
-// Warning: (23-43): Functions in interfaces should be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_default.sol

@@ -2,5 +2,4 @@ interface I {
 	function f();
 }
 // ----
-// Warning: (15-28): Functions in interfaces should be declared external.
-// Warning: (15-28): No visibility specified. Defaulting to "public". In interfaces it defaults to external.
+// TypeError: (15-28): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_internal.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() internal;
 }
 // ----
-// TypeError: (15-37): Functions in interfaces cannot be internal or private.
+// TypeError: (15-37): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_private.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() private;
 }
 // ----
-// TypeError: (15-36): Functions in interfaces cannot be internal or private.
+// TypeError: (15-36): Functions in interfaces must be declared external.

test/libsolidity/syntaxTests/visibility/interface/function_public.sol

@@ -2,4 +2,4 @@ interface I {
 	function f() public;
 }
 // ----
-// Warning: (15-35): Functions in interfaces should be declared external.
+// TypeError: (15-35): Functions in interfaces must be declared external.