Make AM registers into an array

lib/include/am.h

@@ -6,8 +6,24 @@
 
 #define AM_STACK_SIZE 128U
 
+#define AM_ARGL(am) am->regs[ARGL]
+#define AM_ENV(am) am->regs[ENV]
+#define AM_EXPR(am) am->regs[EXPR]
+#define AM_UNEV(am) am->regs[UNEV]
+#define AM_VAL(am) am->regs[VAL]
+
+typedef enum {
+	ARGL,
+	ENV,
+	EXPR,
+	UNEV,
+	VAL,
+
+	AM_REG_COUNT,
+} am_reg_t;
+
 typedef struct am {
-	expr_t *argl, *env, *expr, *unev, *val;
+	expr_t *regs[AM_REG_COUNT];
 	expr_t **sp, *stack[AM_STACK_SIZE];
 	store_t store;
 } am_t;