Bootstrap script improvements

[Akuli]

• Save all bootstrapped compilers. Commit hash is included in folder name, so that this will work even if commits are removed from the list (unlikely).
• When a new commit is added to the bootstrap process, don't recompile everything from scratch.
• For the first commit, use the compiler written in C directly instead of compiling a compiler written in Jou with it. (failed on Windows for some reason)

Fixes #711