minicbor-lease adapter for encoding straight into a lease

[hawkw]

Based on a suggestion from @mkeeter, this branch adds an adapter that implements minicbor::encode::write::Write for a cursor into a writable lease. This lets us avoid double-buffering in the read_ereports path, where we would previously encode each ereport into the receive buffer, and then copy the contents of that into the lease if there is space for it. The new approach avoids the memcpy, and also doesn't encode the ereport in the case where there isn't space left for it.

I implemented the lease-writer adapter thing in a separate minicbor-lease crate, as I was hoping it would be useful elsewhere...but I'm not actually sure if it will be, now that I think of it: the IPC for delivering an ereport to packrat leases the data from the caller, so callers will just encode into a normal buffer they own. Ah well.

[hawkw]

@mkeeter Not sure whether you'd prefer to review this as its own PR or if I should just go ahead and merge it straight into #2126 --- whatever you'd prefer works for me!

[mkeeter]

I'd vote to keep it separate! Review to come (either this afternoon or Monday)

[hawkw]

@mkeeter opened an upstream issue to make the minicbor API less annoying: twittner/minicbor#36