Internal macros should be marked internal in some way

[msimberg]

Macros that are not part of the public API should be recognizable by their name since macros are not namespaced. The primary candidate is PIKA_DETAIL as a prefix since we're already using detail for namespaces. However, something shorter would also be nice, such as PIKA_IMPL. Renaming detail namespaces to impl is also an option since detail is anyway not public.