They belong to internal folder to follow our guideline, not in source as they were.

This PR makes the bare-metal mode work automatically
regardless of the tickless-from-us-ticker setting
- in non RTOS mode
- and in RTOS tickless mode

Chrono conversions inadvertantly changed the core timed sleep routine
used by the RTOS idle to use `OsTimer::update_and_get_tick()` instead of
`OsTimer::get_tick()`.

Correct this, and expand/clarify documentation and naming to try to
prevent recurrence.

Another minor fix observed while inspecting code - `OsClock` can't just
use `milliseconds`, it should match the period of `OsTimer`, which
theoretically can be different.

Remove checking that vectors were copied to RAM as not all targets
have NVIC_RAM_VECTOR_ADDRESS defined as public. Instead always
call NVIC_SetVector unconditionally as old implementation does.