Sol -> Yul: Implement explicit cast

test/libsolidity/semanticTests/viaYul/explicit_cast_assignment.sol

@@ -0,0 +1,10 @@
+contract C {
+    function f() public pure returns (uint16 x) {
+        uint8 y = uint8(0x12345678);
+        x = y;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f() -> 0x78

test/libsolidity/semanticTests/viaYul/explicit_cast_function_call.sol

@@ -0,0 +1,12 @@
+contract C {
+    function f(bytes32 b) public pure returns (bytes32 x) {
+        x = b;
+    }
+    function g() public pure returns (bytes32 x) {
+        x = f(bytes4(uint32(0x12345678)));
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// g() -> 0x1234567800000000000000000000000000000000000000000000000000000000

test/libsolidity/semanticTests/viaYul/explicit_cast_local_assignment.sol

@@ -0,0 +1,10 @@
+contract C {
+    function f(uint a) public pure returns (uint8 x) {
+        uint8 b = uint8(a);
+        x = b;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f(uint256): 0x12345678 -> 0x78