Remove RestartStep from the Dagrt language; some DAGCode changes

This change removes RestartStep, as it was meant to be equivalent to SwitchPhase(<name of current phase>).

The CodeBuilder restart_step() method is kept for convenience and backwards compatibility.

This change also fixes the SwitchPhase code lowering in Fortran to exit immediately.

Additional changes (fallout from the fact that the phase name becomes significant to the CodeBuilder):

• Adds a "name" attribute to ExecutionPhase
• Makes the CodeBuilder constructor arg mandatory
• Renames the CodeBuilder constructor arg from "label" to "name", to fit with the name attribute
• Adds a DAGCode.from_phases_list() classmethod
• Removes old DAGCode classmethods that take statement lists, as they don't fit with the new interface

The language version is bumped to 2019.4.

• Also see leap!28 (merged) for corresponding changes in leap