gottar: converts gotfs to/from tar (#47)

Adds metrics package for reporting progress. (#40)

- metrics package with collectors for metrics, and a context based API for instrumenting code with minimal intrusion from the metrics library.
- TTYRenderer for live view of metrics as they update.
- Calls to metrics have replaced logging as the primary mechanism for communicating progress to an interactive user.  Logging will still have a place in the server processes (e.g. serve, serve-quic).
- package `units` provides unit and SI Prefix constants and utilities.

ptree: index copying (#23)

Adds index copying to `ptree.Copy` function.  Copying indexes makes `Copy` `O(log(n))` where `n` is the number of nodes in the tree.

README: fix spelling