[fine] Improvements to classes

- Classes are defined lazily
- Member access is via environment
- Member access is just a binary expression with a weird environment
- Slot loads look like variable loads now
- Associativity in the parser (ugh)

fine/tests/expression/errors/wild_member_access.fine

@@ -0,0 +1,18 @@
+class Foo {
+  foo: f64;
+}
+
+fun test() {
+  let f = new Foo { foo: 12 };
+  let z = f.{let y = 222; foo };
+}
+
+// NOTE: The AST allows for generic expressions to the right of the dot.
+//       We need to make sure things are parsed correctly.
+//
+// TODO: Better parser recovery will improve the specifics of the errors.
+// @expect-errors:
+// | 7:12: Error at '{': expected an expression
+// | 7:13: Error at 'let': expect ';' to end a let statement
+// | 7:26: cannot find value foo here
+// | 8:0: Error at '}': unbalanced '}'