Keep several versions of spike

[cathales]

Hello!

When you are on master and you go back in the git history, you have to build the old spike version. When you go back to master, you have to build again the version of spike you were using just before.

What would you think about keeping versions of spike in distinct folders? It would allow to re-use previous builds of spike. There would not be "please rm -r spike" error messages anymore; instead it would simply re-use the install directory of the required version or build a new version in a new directory.

To consume less disk usage, we would keep only the install directory, not all the intermediate build artifacts.