feat: Improve UI messages during build and sample
feat: Improve UI messages during build and sample

Improve user interface messages during build and sample.  Report build
progress. Sampling progress reports now resemble those you would see
when doing `git clone` (for a large repository).

These changes make use of poorly-documented features in `clikit`.
Since `clikit` is used by `poetry`, relying on these features seems
safe.

Closes #190

stan-dev/pystan,stan-dev/pystan