Start moving embedding API towards "slot" register-API.

- wrenGetArgumentCount() -> wrenGetSlotCount()
- wrenGetArgument___() -> wrenGetSlot___()

Also, the get functions assert that the value is the right type instead
of checking at runtime. This puts the onus on the caller to be safe,
but maximizes performance.

test/api/benchmark.c

@@ -5,17 +5,18 @@
 static void arguments(WrenVM* vm)
 {
   double result = 0;
-  result += wrenGetArgumentDouble(vm, 1);
-  result += wrenGetArgumentDouble(vm, 2);
-  result += wrenGetArgumentDouble(vm, 3);
-  result += wrenGetArgumentDouble(vm, 4);
-  
+
+  result += wrenGetSlotDouble(vm, 1);
+  result += wrenGetSlotDouble(vm, 2);
+  result += wrenGetSlotDouble(vm, 3);
+  result += wrenGetSlotDouble(vm, 4);
+
   wrenReturnDouble(vm, result);
 }
 
 WrenForeignMethodFn benchmarkBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Benchmark.arguments(_,_,_,_)") == 0) return arguments;
-  
+
   return NULL;
 }

test/api/foreign_class.c

@@ -18,15 +18,15 @@ static void counterAllocate(WrenVM* vm)
 
 static void counterIncrement(WrenVM* vm)
 {
-  double* value = (double*)wrenGetArgumentForeign(vm, 0);
-  double increment = wrenGetArgumentDouble(vm, 1);
+  double* value = (double*)wrenGetSlotForeign(vm, 0);
+  double increment = wrenGetSlotDouble(vm, 1);
 
   *value += increment;
 }
 
 static void counterValue(WrenVM* vm)
 {
-  double value = *(double*)wrenGetArgumentForeign(vm, 0);
+  double value = *(double*)wrenGetSlotForeign(vm, 0);
   wrenReturnDouble(vm, value);
 }
 
@@ -34,9 +34,9 @@ static void pointAllocate(WrenVM* vm)
 {
   double* coordinates = (double*)wrenAllocateForeign(vm, sizeof(double[3]));
 
-  // This gets called by both constructors, so sniff the argument count to see
+  // This gets called by both constructors, so sniff the slot count to see
   // which one was invoked.
-  if (wrenGetArgumentCount(vm) == 1)
+  if (wrenGetSlotCount(vm) == 1)
   {
     coordinates[0] = 0.0;
     coordinates[1] = 0.0;
@@ -44,23 +44,23 @@ static void pointAllocate(WrenVM* vm)
   }
   else
   {
-    coordinates[0] = wrenGetArgumentDouble(vm, 1);
-    coordinates[1] = wrenGetArgumentDouble(vm, 2);
-    coordinates[2] = wrenGetArgumentDouble(vm, 3);
+    coordinates[0] = wrenGetSlotDouble(vm, 1);
+    coordinates[1] = wrenGetSlotDouble(vm, 2);
+    coordinates[2] = wrenGetSlotDouble(vm, 3);
   }
 }
 
 static void pointTranslate(WrenVM* vm)
 {
-  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
-  coordinates[0] += wrenGetArgumentDouble(vm, 1);
-  coordinates[1] += wrenGetArgumentDouble(vm, 2);
-  coordinates[2] += wrenGetArgumentDouble(vm, 3);
+  double* coordinates = (double*)wrenGetSlotForeign(vm, 0);
+  coordinates[0] += wrenGetSlotDouble(vm, 1);
+  coordinates[1] += wrenGetSlotDouble(vm, 2);
+  coordinates[2] += wrenGetSlotDouble(vm, 3);
 }
 
 static void pointToString(WrenVM* vm)
 {
-  double* coordinates = (double*)wrenGetArgumentForeign(vm, 0);
+  double* coordinates = (double*)wrenGetSlotForeign(vm, 0);
   char result[100];
   sprintf(result, "(%g, %g, %g)",
       coordinates[0], coordinates[1], coordinates[2]);

test/api/value.c

@@ -6,7 +6,7 @@ static WrenValue* value;
 
 static void setValue(WrenVM* vm)
 {
-  value = wrenGetArgumentValue(vm, 1);
+  value = wrenGetSlotValue(vm, 1);
 }
 
 static void getValue(WrenVM* vm)