resolve how to handle constants and default binding modes

[nikomatsakis]

Another interesting question that @tschottdorf encountered when implementing default binding modes: What do we do with constants? The RFC specifies that we ought to treat a FOO binding that resolves to a constant as something which can skip through &T types -- however, that runs into trouble if the type of the constant itself is &str or &[T]. The current logic at least skips through all &T or &mut T types when it skips through any, but handling &str correctly would require skipping through "only the right number".

@tschottdorf implemented various rules but we should at minimum update the RFC to match.

[nikomatsakis]

An example. Per the language in the RFC, constants of reference type will not "autoderef", but that means that this example does not work, which seems unfortunate:

let s: &'static str = "abc";
match &s {
  "abc" => true,
  _ => panic!(),
};