Massively simplify and equalise primitive commands

[MattWindsor91]

NOTE: In typical me fashion, this is based on a previous pull request (#142), and shouldn't be merged (or really looked at in detail) until that one is up.

This is a fairly heavy PRQ (got a bit carried away with simplification, as usual) that does a lot of simplifying and expressivity-boosting changes to the primitive commands (assignment, postfix, assume, etc).

What does this do?

• Atomic assignments and postfixes can now mention thread-local variables anywhere. (I think they're still restricted inside indices to only using thread-local variables, but I didn't check as this isn't a problem yet.)
• Microcode and PrimCommands have been merged (sort of): Microcode can now contain stored commands (atomic function calls), and does so until the point where we used to expand out PrimCommands. PrimCommand is now a type alias for a certain type of Microcode. This makes Microcode the only internal representation of Starling's command language.
• Internally, every stored command other than BCAS and ICAS is gone, and the modeller models postfixes, assumes, and id-transitions directly as microcode.
• Local commands now use the exact same AST and internal representation as atomics. The only difference between the two, now, is that local commands can only name thread-local variables. This means commands like {| foo() |} local++ {| bar() |} are now possible.
• Added a load more tests.
• Fixed the weirdly specified CAS in #142.

[MattWindsor91]

@septract I've rebased this onto master now, so it should be ok to look at. Less priority than the last PRQ though.

(Also, I wonder if Travis will work today…)