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