[doc] fix typos in attr.dtx
draft user whatsit allocator
drop calls to deprecated `module()`
avoid shadowing locals
store user whatsits by package
emit warning when defining anonymous whatsits
emit warning after 2^53 user whatsits
accept node as argument for function ``get_user_whatsit_name``
add warning if ``get_user_whatsit_name`` is called with unknown id

another of Stephan Hennig’s suggestions
Merge pull request #5 from phi-gamma/master

drop `module()`; incorporate Stephans’s suggestions