Rename statement classes for paper
I propose the following statement renamings for the paper:
AssignExpression -> Assign
AssignSolved -> AssignImplicit
Reason: "AssignExpression" sounds like a type of expression, so I would avoid that. Also, I don't think the prior names capture the relationship between the two statements well. One option would be to call them AssignExplicit/AssignImplicit, but I think AssignExplicit is too complicated of a name, so I shortened it to Assign.
ExitStep -> RestartStep
Reason: "Exit" misleadingly implies termination
PhaseTransition -> SwitchPhase
Reason: Isn't a verb, like the other statements
I've made the necessary changes and bumped the version to 2019.1