Nested mapgen structures

[Coolthulhu]

Ability to nest mapgen works as follows:

• A special subtype of json mapgen is defined
• This nesteable mapgen json is named
• A mapping or placement is defined on other mapgen to use this new nestable mapgen
• When the mapping or placement is applied, a recursive mapgen call is performed as the last step of the mapgen structure with the mapping/placement

I added an example of this happening. The example is pretty rare in the wild, so during testing it would be a good idea to bump its weight.

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Things that can be achieved with nested mapgen (not done in this PR):

• Create a safe filled with stuff on one random tile in the house. Can't be done with non-nested mapgen as the tile for safe and contents is rolled separately
• Spawn either a human corpse or a zombie, but not both nor neither
• Generate a small structure (say, a doghouse) at random spot in the square instead of being predefined to always spawn at certain distance from the house
• Spawn a corpse with a blood trail to it (like the drug dealers special, but entirely in json) on a random position
• Add a themed set of features, like in the example in this PR (I won't spoil here, read the changes if curious)
• 4 random rooms with a common "hub", like in the labs

[vache]

I like it! Unrelated to the current PR, but on a similar note, I would like to see something similar for specials as well, like with a 5x5 lab structure, but there could potentially be multiple 2x2 "set pieces" that could spawn inside of the main structure.

Like:

LLLLL
LSSLL
LSSLL
LLLLL
LLLLL

SS
SS

could be a large storage area or a cafeteria, depending on how it rolled