README.markdown: add note about jit.ABI_*

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Prefix anonymous type fields in closures with %

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Fix two potential reference leaks when caching

When we fail to weakly cache the instance of a type wrapper in the
wrapper's constructor, we have to make get rid of the strong reference
before forwarding the exception.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>