Now that all of the core functionality for anchor outputs has landed,
we're ready to remove the config flag that was temporarily hiding it
from our API.
This change modifies six structs that were keeping
track of anchors features with an `opt_anchors` field,
as well as another field keeping track of nonzero-fee-
anchor-support.
Rather than using the std benchmark framework (which isn't
maintained and is unlikely to get any further maintenance), we swap
for criterion, which at least gets us a variable number of test
runs so our benchmarks don't take forever.
We also fix the RGS benchmark to pass now that the file in use is
stale compared to today's date.
