ghc-8.6 compatibility

[claudeha]

There are a few things that need to be fixed for ghc-8.6:

• -XNoStarIsType -- otherwise GHC thinks a * b is using the * as type, not type operator. Also need to import Data.Kind (Type) to replace real uses of * as type
• -XUndecidableInstances -- otherwise GHC complains about nested KnownNat (a * b)
• containers-0.6
• hmatrix-0.19

I need to check how best to handle older GHC versions with these changes, will do a pull request soon - how far back do you want to support?

[erikd]

This has been fixed by #85 !