Convert from `log` to `tracing`

Arguably, libraries should be using tracing rather than log to report events. There seem to be bridges between the two.

changed milestone to %A2: Experimental embedded client

mentioned in issue #96 (closed)

This may be necessary for an earlier milestone, if we need it for any particular purpose.

changed milestone to %Arti: Feature parity with the C implementation

changed time estimate to 16h

added Backlog label

mentioned in merge request !64 (merged)

mentioned in commit 44d1271c

Closed by !64 (merged)

closed

mentioned in issue #753