Separate maps.asm, pics.asm, sprites.asm, and tilesets.asm from main.asm (#251)

Each new file builds its own .o, along with separate main.o and home.o, which necessitates many more "exported::" labels.

text/item_names.asm

@@ -1,4 +1,4 @@
-ItemNames:
+ItemNames::
 	db "MASTER BALL@"
 	db "ULTRA BALL@"
 	db "GREAT BALL@"

text/monster_names.asm

@@ -1,4 +1,4 @@
-MonsterNames:
+MonsterNames::
 	db "RHYDON@@@@"
 	db "KANGASKHAN"
 	db "NIDORAN♂@@"

text/trainer_names.asm

@@ -1,4 +1,4 @@
-TrainerNames:
+TrainerNames::
 	db "YOUNGSTER@"
 	db "BUG CATCHER@"
 	db "LASS@"