Initial commit

An assortment of instructions have been implemented so far, mostly basic
stuff such as stack management. The following program can be executed:

int main(void)
{
    hello(2);
    for (;;) {
        system.Peek();
        system.Sleep(1);
    }
    return 0;
}

void hello(int n)
{
    int i;
    for (i = 0; i < n; i++) {
        system.Output("Hello, world! " + string(i));
    }
}

string.c

@@ -0,0 +1,62 @@
+/* Copyright (C) 2019 Nunuhara Cabbage <nunuhara@haniwa.technology>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://gnu.org/licenses/>.
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include "system4.h"
+#include "sys4_string.h"
+
+struct string make_string(char *str, size_t len)
+{
+	return (struct string) {
+		.str = str,
+		.len = len,
+		.mutable = false
+	};
+}
+
+void free_string(struct string *str)
+{
+	if (str->mutable)
+		free(str->str);
+}
+
+void string_add(struct string *a, struct string *b)
+{
+	if (a->mutable) {
+		a->str = xrealloc(a->str, a->len + b->len + 1);
+	} else {
+		char *literal = a->str;
+		a->str = xmalloc(a->len + b->len + 1);
+		memcpy(a->str, literal, a->len + 1);
+	}
+	memcpy(a->str + a->len, b->str, b->len + 1);
+	a->len = a->len + b->len;
+}
+
+struct string integer_to_string(int n)
+{
+	char buf[513];
+	int len = snprintf(buf, 512, "%d" ,n);
+	buf[512] = '\0';
+
+	return (struct string) {
+		.str = xstrdup(buf),
+		.len = len,
+		.mutable = true
+	};
+}