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'.

lib/cretonne/src/ir/mod.rs

@@ -22,7 +22,7 @@ pub use ir::extfunc::{Signature, ArgumentType, ArgumentExtension, ArgumentPurpos
 pub use ir::types::Type;
 pub use ir::entities::{Ebb, Inst, Value, StackSlot, JumpTable, FuncRef, SigRef};
 pub use ir::instructions::{Opcode, InstructionData, VariableArgs, ValueList, ValueListPool};
-pub use ir::stackslot::StackSlotData;
+pub use ir::stackslot::{StackSlotKind, StackSlotData};
 pub use ir::jumptable::JumpTableData;
 pub use ir::valueloc::{ValueLoc, ArgumentLoc};
 pub use ir::dfg::{DataFlowGraph, ValueDef};