Expose APIs to conditionally construct Interface instances

[udoprog]

Use this interface to:

• Implement a string_debug protocol for HashSet and HashMap.
• Implement the eq protocol for HashSet (HashMap can't be
  implemented yet, because Value comparisons for it are not available).

This means that you can now correctly debug format an instance of HashMap and HashSet, even though they are external types:

use std::collections::HashMap;

struct Foo;

pub fn main() {
    let m = HashMap::new();
    m.insert(0, true);
    m.insert(1, Foo);
    dbg(m);
}

Correctly prints:

{0: true, 1: Foo}

Furethermore, HashSet (and soon HashMap) can not be checked for equality:

use std::collections::HashSet;

pub fn main() {
    let a = HashSet::new();
    let b = HashSet::new();
    b.insert(10);
    assert_eq!(a, b);
}

Note: due to limitations, we currently have to use a temporary buffer in the fmt::Debug impl of Value. This can be fixed by making the debug formatting more explicit.