Implemented parsing for UserDefinedValueType

Also added parsing tests.

test/libsolidity/syntaxTests/parsing/user_defined_value_type.sol

@@ -0,0 +1,5 @@
+type MyInt is uint;
+
+contract C {
+    type MyAddress is address;
+}

test/libsolidity/syntaxTests/parsing/user_defined_value_type_err.sol

@@ -0,0 +1,3 @@
+type(MyInt) is uint256;
+// ----
+// ParserError 2314: (4-5): Expected identifier but got '('

test/libsolidity/syntaxTests/parsing/user_defined_value_type_in_function_err.sol

@@ -0,0 +1,6 @@
+function f() {
+    type(uint).max;
+    type MyInt is int;
+}
+// ----
+// ParserError 2314: (44-49): Expected ';' but got identifier