Turn `I`, `left`, `right` into keywords

Reasons:

• They are crucial to the type theory
• Their types are simple enough. The ability to write down prim arcoe : blabla makes the theory easier to understand (if it’s a keyword, you may forget about its type signature, like which parameter is implicit, etc.), but for I, left, right they’re too simple
• My upcoming core type theory refactoring can be simplified drastically using this
• left and right are also used as patterns. By making them keywords we no longer need prim states in pattern checker.

[imkiva]

So we can also simplify lots of code in #256

[ice1000]

@dark-flames Yes