support halting + fast-forwarding the scheduler on halt

this improves performance dramatically in games that utilize halting, including pokemon ruby and golden sun. in both games, framerate was doubled or trippled in many cases

src/crab/cpu.cr

@@ -136,8 +136,10 @@ class CPU
       else
         arm_execute instr
       end
+      @gba.scheduler.tick 1
+    else
+      @gba.scheduler.fast_forward
     end
-    @gba.tick 1
   end
 
   def check_cond(cond : Word) : Bool

src/crab/gba.cr

@@ -83,8 +83,4 @@ class GBA
       cpu.tick
     end
   end
-
-  def tick(cycles : Int) : Nil
-    scheduler.tick cycles
-  end
 end

src/crab/mmio.cr

@@ -67,7 +67,11 @@ class MMIO
       mask = 0xFF00_u16 >> shift
       @waitcnt.value = (@waitcnt.value & mask) | value.to_u16 << shift
     elsif io_addr == 0x301
-      @gba.cpu.halted = bit?(value, 7)
+      if bit?(value, 7)
+        abort "Stopping not supported"
+      else
+        @gba.cpu.halted = true
+      end
     elsif not_used? io_addr
     else
       puts "Unmapped MMIO write ~ addr:#{hex_str index.to_u32}, val:#{hex_str value}".colorize(:yellow)

src/crab/scheduler.cr

@@ -63,4 +63,9 @@ class Scheduler
       end
     end
   end
+
+  def fast_forward : Nil
+    @cycles = @next_event
+    call_current
+  end
 end