Move `rtc::marshalled_options_t` and `rtc::marshal()` into the `detail_` subnamespace

[eyalroz]

Users basically never need to use marshalled_options_t. The only sorta-kinda relevant scenario seems to involve printing, and that's taken care of by rtc::render(); see #630 . We can move it, and related code, into the detail_ subnamespace and non will be the wiser.