Build csources only once
Considering csources is now locked, do you think we can build csources just once and maintain it in `~/.choosenim/toolchains`, and anytime we need to bootstrap, we can simply copy over the previously compiled `nim` binary over? It will save a lot of time when working on random commits.

With #173, devel / #head will not need a full bootstrap anymore on updates. But if we implement #55 and #58, any commit / branch will need bootstrap. Having csources cached will be very helpful when working on random PRs and branches.