Add string buffer util

This will help to build strings incrementally.

app/tests/test_strbuf.c

@@ -0,0 +1,47 @@
+#include "common.h"
+
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "util/strbuf.h"
+
+static void test_strbuf_simple(void) {
+    struct sc_strbuf buf;
+    bool ok = sc_strbuf_init(&buf, 10);
+    assert(ok);
+
+    ok = sc_strbuf_append_staticstr(&buf, "Hello");
+    assert(ok);
+
+    ok = sc_strbuf_append_char(&buf, ' ');
+    assert(ok);
+
+    ok = sc_strbuf_append_staticstr(&buf, "world");
+    assert(ok);
+
+    ok = sc_strbuf_append_staticstr(&buf, "!\n");
+    assert(ok);
+
+    ok = sc_strbuf_append_staticstr(&buf, "This is a test");
+    assert(ok);
+
+    ok = sc_strbuf_append_n(&buf, '.', 3);
+    assert(ok);
+
+    assert(!strcmp(buf.s, "Hello world!\nThis is a test..."));
+
+    sc_strbuf_shrink(&buf);
+    assert(buf.len == buf.cap);
+    assert(!strcmp(buf.s, "Hello world!\nThis is a test..."));
+
+    free(buf.s);
+}
+
+int main(int argc, char *argv[]) {
+    (void) argc;
+    (void) argv;
+
+    test_strbuf_simple();
+    return 0;
+}