list_or: chaniging how list_or_* functions work and adding a generic list_or

list_or_* functions (e.g. list_or_string) will now always return a list,
however will accept lists or indivitual values. Also added a list_or()
generator function, similar to what already exists for list_of().

wlauto/instrumentation/perf/__init__.py

@@ -23,7 +23,7 @@ import itertools
 from wlauto import Instrument, Executable, Parameter
 from wlauto.exceptions import ConfigError
 from wlauto.utils.misc import ensure_file_directory_exists as _f
-from wlauto.utils.types import list_or_string, list_of_strs
+from wlauto.utils.types import arguments, list_of_strs
 
 PERF_COMMAND_TEMPLATE = '{} stat {} {} sleep 1000 > {} 2>&1 '
 
@@ -71,7 +71,7 @@ class PerfInstrument(Instrument):
         Parameter('events', kind=list_of_strs, default=['migrations', 'cs'],
                   constraint=(lambda x: x, 'must not be empty.'),
                   description="""Specifies the events to be counted."""),
-        Parameter('optionstring', kind=list_or_string, default='-a',
+        Parameter('optionstring', kind=arguments, default='-a',
                   description="""Specifies options to be used for the perf command. This
                   may be a list of option strings, in which case, multiple instances of perf
                   will be kicked off -- one for each option string. This may be used to e.g.