Rely on YARD, not the shell, to expand lib/**/*.rb glob

Unless the globstar shopt is set, shells won't expand lib/**/*.rb by
default. This means that lib/*.rb files won't be included in the
lib/**/*.rb glob.

YARD will expand globs it sees.
Split up stdlib target to allow regeneration without redownloading
Explain tests better, using paths from PR #2
update Dockerfile to use new rvm-independent `make stdlib`
Update program expected output