Add string buffer util

This will help to build strings incrementally.

app/src/util/strbuf.h

@@ -0,0 +1,37 @@
+#ifndef SC_STRBUF_H
+#define SC_STRBUF_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <string.h>
+
+struct sc_strbuf {
+    char *s;
+    size_t len;
+    size_t cap;
+};
+
+// buf->s must be manually freed by the caller
+bool
+sc_strbuf_init(struct sc_strbuf *buf, size_t init_cap);
+
+bool
+sc_strbuf_append(struct sc_strbuf *buf, const char *s, size_t len);
+
+bool
+sc_strbuf_append_char(struct sc_strbuf *buf, const char c);
+
+bool
+sc_strbuf_append_n(struct sc_strbuf *buf, const char c, size_t n);
+
+static inline bool
+sc_strbuf_append_str(struct sc_strbuf *buf, const char *s) {
+    return sc_strbuf_append(buf, s, strlen(s));
+}
+
+// Append static string (i.e. the string size is known at compile time, for
+// example a string literal)
+#define sc_strbuf_append_staticstr(BUF, S) \
+    sc_strbuf_append(BUF, S, sizeof(S) - 1)
+
+#endif