runtime: move r5js.ProcCall to its own file.
runtime: run proc.js through linter.
runtime: make r5js.ProcCall#isSpecialOperator_ private.
runtime+ make r5js.ProcCall a @struct.
runtime: add+use an Assignment subclass of r5js.ProcCall.
runtime: add+use an IdShim subclass of r5js.ProcCall.
misc: rename newIdShim => r5js.procs.newIdShim.
runtime: rename r5js.procs.newAssignment => r5js.Assignment.newContinuable.
runtime: don't export r5js.Assignment.
misc: rename r5js.procs.newIdShim => r5js.newIdShim. Don't export IdShim.
runtime: move two fields from r5js.ProcCall to Assignment subclass.