[fine] Assignments!

And new error capabilities!

fine/tests/expression/errors/assignment_errors.fine

@@ -0,0 +1,20 @@
+fun something() { }
+fun something_else() { }
+
+fun wrong() {
+  let x = 12;
+  let y = "hello";
+  x = y; // should be an error
+  y = x; // should be an error
+
+  let z = 23;
+  y = x = z; // should be an error
+
+  something_else = something; // should be an error
+}
+
+// @expect-errors:
+// | 7:4: cannot assign a value of type `string` to type `f64`
+// | 8:4: cannot assign a value of type `f64` to type `string`
+// | 11:4: cannot assign a value of type `f64` to type `string`
+// | 13:2: cannot assign a new value to a function declaration