Add bytes type and tests

bytes_t is a local (automatic) allocation array that carries the length
and capacity with it.

src/bytes.c

@@ -0,0 +1,6 @@
+#include "bytes.h"
+#include "error.h"
+
+error_t *const err_bytes_no_capacity = &(error_t){
+    .message = "Not enough capacity in bytes buffer",
+};

src/bytes.h

@@ -0,0 +1,60 @@
+#ifndef INCLUDE_SRC_BYTES_H_
+#define INCLUDE_SRC_BYTES_H_
+
+#include "error.h"
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+extern error_t *const err_bytes_no_capacity;
+
+typedef struct bytes {
+    size_t len;
+    size_t cap;
+    uint8_t buffer[];
+} bytes_t;
+
+#define LOCAL_BYTES_ANONYMOUS(N)                                               \
+    &(struct {                                                                 \
+        size_t len;                                                            \
+        size_t cap;                                                            \
+        uint8_t buffer[(N)];                                                   \
+    }) {                                                                       \
+        0, (N), {}                                                             \
+    }
+
+#define LOCAL_BYTES(N) (bytes_t *)LOCAL_BYTES_ANONYMOUS(N);
+
+static inline error_t *bytes_append_uint8(bytes_t *bytes, uint8_t value) {
+    if (bytes->len >= bytes->cap)
+        return err_bytes_no_capacity;
+    bytes->buffer[bytes->len++] = value;
+    return nullptr;
+}
+
+static inline error_t *bytes_append_array(bytes_t *dst, size_t n,
+                                          uint8_t buffer[static n]) {
+    if (dst->len + n >= dst->cap)
+        return err_bytes_no_capacity;
+    memcpy(dst->buffer + dst->len, buffer, n);
+    dst->len += n;
+    return nullptr;
+}
+
+static inline error_t *bytes_append_bytes(bytes_t *dst, bytes_t *src) {
+    return bytes_append_array(dst, src->len, src->buffer);
+}
+
+static inline error_t *bytes_append_uint16(bytes_t *dst, uint16_t value) {
+    return bytes_append_array(dst, sizeof(value), (uint8_t *)&value);
+}
+
+static inline error_t *bytes_append_uint32(bytes_t *dst, uint32_t value) {
+    return bytes_append_array(dst, sizeof(value), (uint8_t *)&value);
+}
+
+static inline error_t *bytes_append_uint64(bytes_t *dst, uint64_t value) {
+    return bytes_append_array(dst, sizeof(value), (uint8_t *)&value);
+}
+
+#endif // INCLUDE_SRC_BYTES_H_