Add string buffer util

This will help to build strings incrementally.

app/src/util/strbuf.h

@@ -0,0 +1,73 @@
+#ifndef SC_STRBUF_H
+#define SC_STRBUF_H
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <string.h>
+
+struct sc_strbuf {
+    char *s;
+    size_t len;
+    size_t cap;
+};
+
+/**
+ * Initialize the string buffer
+ *
+ * `buf->s` must be manually freed by the caller.
+ */
+bool
+sc_strbuf_init(struct sc_strbuf *buf, size_t init_cap);
+
+/**
+ * Append a string
+ *
+ * Append `len` characters from `s` to the buffer.
+ */
+bool
+sc_strbuf_append(struct sc_strbuf *buf, const char *s, size_t len);
+
+/**
+ * Append a char
+ *
+ * Append a single character to the buffer.
+ */
+bool
+sc_strbuf_append_char(struct sc_strbuf *buf, const char c);
+
+/**
+ * Append a char `n` times
+ *
+ * Append the same characters `n` times to the buffer.
+ */
+bool
+sc_strbuf_append_n(struct sc_strbuf *buf, const char c, size_t n);
+
+/**
+ * Append a NUL-terminated string
+ */
+static inline bool
+sc_strbuf_append_str(struct sc_strbuf *buf, const char *s) {
+    return sc_strbuf_append(buf, s, strlen(s));
+}
+
+/**
+ * Append a static string
+ *
+ * Append a string whose 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)
+
+/**
+ * Shrink the buffer capacity to its current length
+ *
+ * This resizes `buf->s` to fit the content.
+ */
+void
+sc_strbuf_shrink(struct sc_strbuf *buf);
+
+#endif