Allow dumping a weighted list to a string for debug purposes.

Previously when placing an overmap special on top of an existing overmap
special (e.g. via the debug menu) the parameters for the old one would
persist and the new one would lack its parameters.  Instead, overwrite
the old with the new.

We want mapgen parameters to work inside nested mapgen, at all scopes.
To make this work we lift all parameters from nests to overmap terrains
before lifting again from overmap terrains to overmap specials.

At mapgen time, the arguments chosen for these parameters cascade down
in the opposite direction.

A simple bugfix.  Previously we were generating these arguments but not
passing them.

State the two distributions so that it is easier to figure out where
each is coming from and why they are conflicting.

Use the new functionality of mapgen parameters in nests to choose a
random colour of carpet for each carpet used in a nested mapgen using
house_w_nest_palette.

Previously there was a separate symbol for each carpet colour but only 6
(red carpet) was ever used.  Remove the unused symbols and repurpose 6
to mean "random carpet".  Make the parameter of nest scope so each nest
can choose a carpet colour independently.