Split items/ and trainers/ into their data/ and engine/ components

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/poliwag.asm

@@ -0,0 +1,10 @@
+; PoliwagPokedexEntry: ; 0x182f91
+	db "TADPOLE@" ; species name
+	dw 200, 270 ; height, weight
+
+	db   "The swirl on its"
+	next "belly is its"
+	next "insides showing"
+	page "through the skin."
+	next "It looks clearer"
+	next "after it eats.@"