Ore distribution: Deeper iron, diamond and mese block, tune gold (#2107)

Iron at y = 0 was far too easy to find.
Adjust gold lower region to be twice the depth of highest level, like
all other ores.
Diamond at y = -500 was too easy to progress to.
Make diamond and mese block deeper to create a depth progression from
mese crystal to diamond, to reflect tool progression.
This all creates a satisfying 2^n depth progression, with lower regions
being at twice the depth of highest levels.