Move callback_info::invoke out of header, update behavior.
Add node to invoke comment.
Enum formatting.
Add missing cstdint include to _types.h

This got included by virtue of _types.h ending up in files that always
included cstdint already, but it breaks if that doesn't happen in the
right order, obviously.
Missing stdexcept include in vm_exception.h

Required for std::logic_error and such. Never resulted in an error,
which probably just means it escaped by always getting included in just
the right order.
Remove commented out code.
utility include in vm_thread.h.

Probably required elsewhere for std::forward usage as well, though it
likely gets pulled in implicitly by a bunch of other headers, that
shouldn't be relied on.
Three line breaks between non-inline methods.
Move one remaining bind_callback impl. out of header.
Strip out unneeded includes. Move one to implementation.