Implement basic variables (to hide pointers from logs)

shell.h

@@ -3,11 +3,27 @@
 
 #include <stdio.h>
 
+enum shell_var_type {
+    SHELL_VAR_SINT,
+    SHELL_VAR_UINT,
+    SHELL_VAR_POINTER,
+};
+
+#define SHELL_VAR_NAME_LEN 16
+
+struct shell_var {
+    struct shell_var *next;
+    enum shell_var_type type;
+    char name[SHELL_VAR_NAME_LEN];
+    union {ssize_t sint; size_t uint; uint64_t uint64; void *ptr;} value;
+};
+
 struct shell_ctx {
     FILE *fin;
     FILE *fout;
     int reproducible;
     const char *hist_file;
+    struct shell_var *var;
 };
 
 void *run_test(struct shell_ctx *ctx);