Move to F# 6

[pbiggar]

Move up to F# 6. Note that we don't use any features of it yet:

• Ply is faster than F# tasks (with Resumable State Machines) in my tests by about 25% (tested using https://github.com/darklang/fizzboom)
• Fantomas doesn't seem to like formatting the non-dot versions of table lookups (dict[key] vs dict.[key])