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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/aipom.asm

@@ -0,0 +1,10 @@
+; AipomPokedexEntry: ; 0x1cda11
+	db "LONG TAIL@" ; species name
+	dw 207, 250 ; height, weight
+
+	db   "It uses its tail"
+	next "to hang on to tree"
+	next "branches. It uses"
+	page "its momentum to"
+	next "swing from one"
+	next "branch to another.@"