For now, if ONLY_WELL_FORMED_STRINGS_PASSABLE environment option is 'enabled', then assertPassableString is the same as assertWellFormedString. Otherwise assertPassableString just asserts that str is a string.

Currently, ONLY_WELL_FORMED_STRINGS_PASSABLE defaults to 'disabled' because we do not yet know the performance impact. Later, if we decide we can afford it, we'll first change the default to 'enabled' and ultimately remove the switch altogether. Be prepared for these changes.

TODO once the switch is removed, simplify assertPassableString to simply be assertWellFormedString.