Track stack slot kinds.

Add a StackSlotKind enumeration to help keep track of the different
kinds of stack slots supported:

- Incoming and outgoing function arguments on the stack.
- Spill slots and locals.

Change the text format syntax for declaring a stack slot to use a kind
keyword rather than just 'stack_slot'.

filetests/parser/tiny.cton

@@ -96,8 +96,10 @@ ebb0(v90: i32, v91: f32):
 
 ; Stack slot references
 function %stack() {
-    ss10 = stack_slot 8
-    ss2 = stack_slot 4
+    ss10 = spill_slot 8
+    ss2 = local 4
+    ss3 = incoming_arg 4
+    ss4 = outgoing_arg 4
 
 ebb0:
     v1 = stack_load.i32 ss10
@@ -106,8 +108,10 @@ ebb0:
     stack_store v2, ss2
 }
 ; sameln: function %stack() {
-; nextln:    $ss10 = stack_slot 8
-; nextln:    $ss2 = stack_slot 4
+; nextln:    $ss10 = spill_slot 8
+; nextln:    $ss2 = local 4
+; nextln:    $ss3 = incoming_arg 4
+; nextln:    $ss4 = outgoing_arg 4
 
 ; check: ebb0:
 ; nextln: $v1 = stack_load.i32 $ss10