Nuumerous minor improvements.

Improved @{*-core/*-test/*-os} to differentiate between iOS and OS X as well as
between OS traits (e.g., :is_os_trait_darwin()) and actual OSes (e.g.,
:is_os_ios()). Improved :define_function_if() to accept an optional fallback
function body. Merged @{*-core/path/time} and @{*-main/path/time}. Documented
numerous minor improvements to be made as time and surfeit faith avail.