There are some missing parts here (like persistence and tests)
and some incorrect parts (I am 90% sure that the "exploratory
circuit" flag is bogus). Also it is not integrated with the circuit
manager code.
We'll need `id_pair_is_listed()` to track whether a sampled guard is
(or is not) listed in the consensus.
We'll need `missing_descriptor_for` to see whether we've downloaded
enough microdescs to use a consensus.
(Thank goodness for rust; we messed up the coherency in C here so
many times, but I'm pretty sure that this time around we can't have
gotten it wrong.)
Previously, we'd have to declare the field for a parameter in one
place, its default in a second, and its consensus key in a third.
That's error-prone and not so fun! This patch changes the
way we declare parameters so that we declare a structure once,
and macros expand it to all do the right thing.
This required a few new traits and implementations to ensure
uniformity across the types that can go in parameters: We need every
parameter type to implement TryFrom<i32> and to implement
SaturatingFromInt32.
Eventually we might want SaturatingFromInt32 to be a more generic
SaturatingFrom, but that's not for now.
Doing this makes the code faster, lets us throw away some code, and
makes it easier to add a "choose-N-disjoint relays" implementation.
See large comment about plusses and minuses of new code. (Note that
the old implementation wasn't constant-time either.)
Dimitris Apostolou
Nick Mathewson
Daniel Eades