Right now `declare' is essentially a hack; it only works because `def*'
forms recognize and remove it from the lambda which is then passed to
defalias.  And due to the unfortunate order of `declare' and
`interactive' forms in defun it is not possible to simply put the
`declare' into the lambda, as that would break any following
`interactive' forms.

This alone is a simple fix - I have attached a PoC patch - but the
question of what to actually do with those declarations is one that is
very wide-reaching and I cannot answer that, much less implement it, on
my own.

One possibility which I am thinking of would be to add another slot to
closures which holds these declare forms and to make function-get and
function-put refer to these slots instead, and while a lot of
declarations would work out of the box with this setup, stuff like gv's
declarations will not.

A solution might be to have `def*' apply and remove the declarations
they recognize in their `*-declarations-alist' and send the rest over to
the `lambda', which *would* conceptually work, but as I have not
actually implemented it might contain pitfalls that I cannot predict at
the moment.

I would like to get some opinions on this.  I think that at the very
least lambdas should recognize declares even if they don't do anything
with it.