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'.

docs/example.cton

@@ -1,7 +1,7 @@
 test verifier
 
 function %average(i32, i32) -> f32 {
-    ss1 = stack_slot 8            ; Stack slot for ``sum``.
+    ss1 = local 8            ; Stack slot for ``sum``.
 
 ebb1(v1: i32, v2: i32):
     v3 = f64const 0x0.0

docs/langref.rst

@@ -465,9 +465,9 @@ frame. The stack frame is divided into fixed-size stack slots that are
 allocated in the :term:`function preamble`. Stack slots are not typed, they
 simply represent a contiguous sequence of bytes in the stack frame.
 
-.. inst:: SS = stack_slot Bytes, Flags...
+.. inst:: SS = local Bytes, Flags...
 
-    Allocate a stack slot in the preamble.
+    Allocate a stack slot for a local variable in the preamble.
 
     If no alignment is specified, Cretonne will pick an appropriate alignment
     for the stack slot based on its size and access patterns.