CircleCI-based macOS CI

Fixes: #99

Since I don't want to build another right now, drop it.

So optimistic...

Accept tahoe-lafs as an optional parameter

Fall back to our internal packaging if it is not supplied.

Also, shift towards less use of overlays.  Explicit arguments are better when
possible because they don't induce freaky action at a distance.

Make ShareStat actually copyable

Also add a test that actually serializes it

Add an HTTP API for price calculation

Parse the byte string pass-value into an integer when initializing the server plugin

Consider lease state (expired or not) when the client computes the price of a mutable write

Require the Tahoe-LAFS API authorization token to access the ZKAPAuthorizer HTTP API.