this was done to enable .list_locals to effectively test option parsing, and the
option parsing was revised to make use of it. The supporting changes included:
* Executable was divided into Base_executable and Named_executable
* std::vector<Arg_script> was separated out as a Command_block which is itself
  a Base_executable.
* promote_soons() is a modifying function rather than one that always constructs
  a replacement