feature request: rename folder [backend]-nom to [backend]-sha1

[ugexe]

I rebuild rakudo daily, and often find myself wishing I had a copy of the previous version still when I run into certain behaviors or when rakudo fails to build (leaving me with no perl6 installation). Right now I can either:

1) `rakudobrew build moar` && `rakudobrew build moar [last-sha1]'
2) mv ~/.rakudobrew/moar-nom ~/.rakudobrew/moar-[last-sha1]

(1) means rebuilding an install I just delete, including all modules. (2) actually also involves extracting the sha-1 of the old nom for renaming the folder so is less than ideal, and I think renaming the folder would actually break things sometimes.

What I suggest is building all backends into folders with the sha-1 appended (like rakudobrew build moar f1a21af does). Then keep track of the current HEAD similar to how the CURRENT file works, so that you can still map moar-nom or just moar to HEAD. Now when you do rakudobrew build moar and you already have a moar installation it doesn't need its folder name (moar-nom/), instead creating its own folder (moar-[sha1]/). At this point you can default to deleting the old installation or keep it around, and have a flag for doing the other. Maybe this is all better left to an outside script, but I think it may be a nice feature to have integrated