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An "immediate" rather than generative function is also available;
the docs contain caveats.

At the moment, these distinctions are a bit forced, but we want to be
ready for when we start getting truly huge maps where the generative
usage actually *matters* for either memory reasons, latency reasons,
or both.

Separated Length rather than trying to pull double-duty the Keys
method; the previous combination was just utterly silly.

The implementations in the bind package are stubs; that package is
going to take a lot of work, and so the focus is going to be entirely
on keeping the 'free' package viable; and we'll revisit bind and such
when there's more dev time budget.
Signed-off-by: Eric Myhre <hash@exultant.us>