New faults and syntax (#2034)

- Remove `[?]` syntax.
- Change `int!` to `int?` syntax.
- New `fault` declarations.
- Enum associated values can reference the calling enum.

resources/examples/dynlib-test/add.c3

@@ -4,7 +4,7 @@ struct Foo(Printable)
 	int a;
 }
 
-fn usz! Foo.to_format(&self, Formatter* f) @dynamic
+fn usz? Foo.to_format(&self, Formatter* f) @dynamic
 {
 	return f.printf("Foo[%d]", self.a);
 }