inspect.getfullargspec in module constructor causes problems with function wrappers

[jonkhler]

When trying to annotatedd haiku modules with jaxtyping type hints, I realized there is a general problem with function wrappers due to using the (semi-deprecated) inspect.getfullargspec in

https://github.com/deepmind/dm-haiku/blob/2664390c3df8af703b87bf62059eda25ca5eff57/haiku/_src/utils.py#L66

E.g. when you wrap the __init__ of a haiku module with a type checker then this call produces an error due to missing signature information (getfullargspec does not follow the full __wrapped__-chain).

https://github.com/deepmind/dm-haiku/blob/2664390c3df8af703b87bf62059eda25ca5eff57/haiku/_src/module.py#L126

See this PR to jaxtyping. As discussed with @patrick-kidger this seems to be a bug/incompatibility issue at the haiku side rather than forcing wrapper libraries to hard-patch a __signature__ attribute.