[epic] Make caching real good

We have several significant opportunities to improve the ways rv caches things to make the things we do often really fast:

• set up the cacache crate for the other parts of rv to use
• unpack ruby and gem tarballs into our cacache, so we only store each file once even if it is in multiple versions of a gem
• install gems by copy-on-write hardlinking the files in that gem from cacache into the install directory, instead of unpacking tarballs
• after compiling native extensions, copy the results into cacache. on future installs simply hardlink those files as well, so any particular machine will never compile a gem more than once
• put git gem clones into the global cache, so you never have to do a full checkout of a git repo (eg rails) a second time