DeprecatedFlyString relies heavily on DeprecatedString's StringImpl, so
let's rename it to A) match the name of DeprecatedString, B) write a new
FlyString class that is tied to String.
Before this we attempted to hack around this by only overriding
has_binding. However this did not cover all cases, for example when
assigning to variables before their declaration it didn't throw.
By using the new find_binding_and_index virtual method we can just
pretend the indirect bindings are real.
Since indirect binding do come from a normal environment we need to
ensure you cannot modify the binding and that properties like mutable
are false as expected by the spec for such an indirect binding.
