Refactor reading

Major effects: effects preventing you from reading scrolls now
appear in MSGCH_PROMPT instead of the default channel, and
silence, water elementals, and ru's no-scrolls effect no longer
prevent you from reading books with (r). This also means that
those effects are only warned about after you finish choosing a
scroll through (r), rather than when you hit (r). If that seems
annoying, please remove the "read books with (r)" functionality.
Make Blurry Vision cause reading to take longer

Try number two (ref 6df881ba), but this time implemented as a
delay, such that it actually takes longer for e.g. tele to kick
in after you start trying to read the scroll.

Currently takes one extra turn for each level of the mutation
(so 1+1 turns at level 1, 2+1 for level 2, 3+1 for level 3).
Possibly it would be better for the last level to give 4+1.

TODO: Would be good if there was a DELAY_ARMOUR_* style chance to
cancel out of the reading midway through, but I ran into
technical issues with the implementation - its messaging caused
the interrupt to fire immediately every time...?