Fixed recursive async function calls.

Fixed recursive async function calls.

Previously, PromiseCapability record was stored (function->context)
directly in function object during a function invocation.  This is
not correct, because PromiseCapability record should be linked to
current execution context.  As a result, function->context is
overwritten with consecutive recursive calls which results in
use-after-free.

This closes #451 issue on Github.