Fixed copying of closures for declared functions.

Fixed copying of closures for declared functions.

After 0a2a0b5a74f4 (0.6.0),  the referencing of a closure value inside of
a nested function may result in heap-use-after-free.  For this to happen
the closure value have to be referenced in a function invoked asynchronously.
For example if a closure value is referenced in r.subrequest() or setTimeout()
handler.

The problem was that closure values of nested function were assigned during
the function call and the memory shared between all the cloned VMs was used
to store temporary assignments until the moment the declared function was
referenced.  When two VMs executed concurrently, the first VM might see
the changed made by the second VM if the first one was suspended.

The fix is to copy all declared functions at the time of the call.

This closes #421 issue on GitHub.