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

Move some data into a pokemon/ subdirectory

data/pokemon/dex_entries/swinub.asm

@@ -0,0 +1,10 @@
+; SwinubPokedexEntry: ; 0x1d0b8d
+	db "PIG@" ; species name
+	dw 104, 140 ; height, weight
+
+	db   "It uses the tip of"
+	next "its nose to dig"
+	next "for food. Its nose"
+	page "is so tough that"
+	next "even frozen ground"
+	next "poses no problem.@"