Rework ARTP_STEALTH akin to MR rescale

Now Stlth+ stands for a discrete step of +50 stealth, Stlth- for -50 and so on.
Account for ARTP_STEALTH when pricing artefacts
Remove ARTP_METABOLISM

Convert existing ones into a negative level of ARTP_STEALTH.
Replace rings of hunger/sustenance with loudness/stealth in-place

Those give one level of -50/+50 stealth each. Stealth is priced at old
sustenance level (but often comes cursed), loudness around 50-60 after curses.
Rework stealth adjective scale to steps of 50

(And one of 100 at the end.)

I'm not yet fully convinced linearity is better than the old progression.
This does however look not too intrusive, and might enable presentation in form
of a meaningful stealth bar at some point.
Rings/artefacts of stealth (new scale) always affect the stealth description
until 400 stealth. Some other sources could be tweaked a bit to fit in as well.

Then of course there are smaller factors like racial stealth (as for MR).
Together with certain multipliers like from being in water, backlit, encumbered
and so on, changing them to fit into one dead-set scale is neither desired nor
in scope of these changes.