Refactored additional getters into setters.

Specifically, refactored get_integer_random_in_range() to
:set_int_to_random_in_range() and redefined the latter in terms of
:define_function_if() -- which is to say, dynamically. Numerous other minor
improvements abound pertaining to the same drab subject.
Added support for tester-dependent suboverlays.

Specifically, implemented low-level functions in @{compile} handling tester-
dependent suboverlays (e.g., @{if_display_server}, @{if_package_manager}) in a
reasonably generic manner *AND* reimplemented high-level functions compiling
such suboverlays in terms of such low-level functions.

Incidentally, concocted a new @{*-start/*-alias_function} parcel containing
functions wrapped in eponymous aliases (e.g., alias :die_unless_arg_count_even()
wrapping the function of the same name).