Value::iter without collecting into a Vec

[stepancheg]

There are two subtleties in this PR.

First, Rust standard library has no RefVal object which
contains not a reference like Ref but a value bound to RefVal
lifetime. Seems like it is not possible to implement RefVal
safely in current Rust.

Since it's not possible to return anything except references inside
RefCell, returned value is "iterable", not "iterator" (simply
downcast TypedValue).

It's not possible to implement IntoIterator for Ref, so
IntoIterator is implemented for &Ref (&RefIterable).

Thus iteration over value returned by Value::iter now looks like this:

let v: Value = ...
for item in &v.iter()? { ... }

And if an iterator is needed explicitly, iterable need to be stored
in a temporary variable.

let v: Value = ...
let iterble = v.iter()?;
let mut iterator = iter.iter();
iterator.next();

// this is invalid because iterator lifetime is bound to iterable
// let mut iterator = v.iter()?.iter();

Second, mutability flag is hidden inside TypedValue, so to
query mutability we need to borrow TypedValue first. But we need
to check mutability before invoking mutating operations.

So Value has now magic borrow_mut_check_mutability function,
which borrows immutably after borrow_mut failed to return proper
error.

[stepancheg]

This issue d-e-s-o/cell#5 discusses why RefVal is not possible to implement in current Rust.

This WIP PR of mine #94 would allow querying mutability without borrowing.