Improve the way we pick the package-set to upgrade to

[f-f]

As we prepare the 0.14 release of the compiler, it looks like we won't be able to put out a matching package set for some time - this means that people pulling in the toolchain for the first time will get the new compiler and try to use it with the 0.13.8-compatible set, which right now results in a confusing error.

I believe the "proper" fix would be #633, but that's a fairly complex change, while this PR implements a simpler, but smarter logic to pick the set version (see #657 for details) on init and upgrade.
This includes properly warning the user when they use a compiler for which we don't yet have a set out. This is the error message they'd get if trying to use the 0.13.8-compatible set with the newer purs 0.14:

[error] It seems that you're using a compiler version that is not supported by package-sets at the moment.
Please install one of the following versions of the compiler and try again: 
- 0.12.0
- 0.12.1
- 0.12.2
- 0.12.3
- 0.12.4
- 0.12.5
- 0.13.0
- 0.13.2
- 0.13.3
- 0.13.4
- 0.13.5
- 0.13.6
- 0.13.8

Fix #687, fix #657, fix #666

[thomashoneyman]

Should we reverse this list so that recent compiler versions are listed first?

[f-f]

@thomashoneyman excellent idea 👏