Deprecate `&` in favour of `#`

[i-am-the-slime]

& is very useful for defining hooks in the order they appear in the code and with fewer parens. However, there is import Type.Function (type (#)) which does the same and is consistent with # on the value level.

I think it would be a good idea to put a user defined warning on & and tell users to switch to # for a while and eventually deprecate &.