A flood of minmay vaults (#10368, #10423)

Of the many submissions of mostly D vaults, probably the most noticeable
are the two sets which got their own files and functions: d:1-2 symmetrical
polyominos using a few floor spawns, and highly dangerous large subvault
grids for D:7- / Elf / Depths.

Select monster placements with highly random potential threat density were
constrained away from extremely rare low and high points with NSUBST instead
of SUBST. Close-together decorative statues in excess of, say, fifteen per
vault, were trimmed down, due to concerns of over-use and visual noise in
tiles. Vault-wide crystal use chances were also lowered. Eventually I'm going
to just break down and impose certain styles and standards onto absolutely
every single last vault, I guess.

I also gave some elementally-themed vaults variations for Cocytus and Gehenna,
which removes the items and places a fiend + a staircase instead.