ObjDefManager, Mapgen SAPI: Huge refactoring

- General code cleanup
 - Unified object creation and loading
 - Specifying objects in APIs is now orthogonal (i.e. anything can take an ID,
   name string, or the raw table definition (and automatically registers if present

doc/lua_api.txt

@@ -3195,6 +3195,7 @@ Definition tables
         biomes = {"Oceanside", "Hills", "Plains"},
     --  ^ List of biomes in which this decoration occurs.  Occurs in all biomes if this is omitted,
     --  ^ and ignored if the Mapgen being used does not support biomes.
+    --  ^ Can be a list of (or a single) biome names, IDs, or definitions.
         y_min = -31000
         y_max = 31000
     -- ^ Minimum and maximum `y` positions these decorations can be generated at.
@@ -3221,6 +3222,7 @@ Definition tables
         schematic = "foobar.mts",
     --  ^ If schematic is a string, it is the filepath relative to the current working directory of the
     --  ^ specified Minetest schematic file.
+    --  ^  - OR -, could be the ID of a previously registered schematic
     --  ^  - OR -, could instead be a table containing two mandatory fields, size and data,
     --  ^ and an optional table yslice_prob:
         schematic = {