In SpyReader, don't return pointers into main content string

This improves test coverage of the lexer. Before, a SpyReader's read function
would return pointers into a single string that contained the entire text. This
could have masked bugs where out-of-bounds characters were being read.
Now the chunks returned by the reader are copied into a separate buffer.

spec/runtime/helpers/spy_reader.cc

@@ -5,23 +5,23 @@ using std::string;
 
 static const char * spy_read(void *data, size_t *bytes_read) {
   SpyReader *reader = static_cast<SpyReader *>(data);
-  size_t size = std::min(reader->chunk_size,
-                         reader->content.length() - reader->position);
-  const char *result = reader->content.data() + reader->position;
-  reader->strings_read.back() += string(result, size);
-  reader->position += size;
-  *bytes_read = size;
-  return result;
+  string result = reader->content.substr(reader->position, reader->chunk_size);
+  reader->position += result.size();
+  reader->strings_read.back() += result;
+  *bytes_read = result.size();
+  memcpy(reader->buffer, result.data(), result.size());
+  return reader->buffer;
 }
 
-static int spy_seek(void *data, size_t position) {
+static int spy_seek(void *data, TSLength position) {
   SpyReader *reader = static_cast<SpyReader *>(data);
   reader->strings_read.push_back("");
-  reader->position = position;
+  reader->position = position.bytes;
   return 0;
 }
 
 SpyReader::SpyReader(string content, size_t chunk_size) :
+    buffer(new char[chunk_size]),
     content(content),
     position(0),
     chunk_size(chunk_size),
@@ -35,3 +35,7 @@ SpyReader::SpyReader(string content, size_t chunk_size) :
 void SpyReader::clear() {
   strings_read.clear();
 }
+
+SpyReader::~SpyReader() {
+  delete buffer;
+}

spec/runtime/helpers/spy_reader.h

@@ -8,9 +8,11 @@
 class SpyReader {
  public:
   SpyReader(std::string content, size_t chunk_size);
+  ~SpyReader();
 
   void clear();
 
+  char *buffer;
   std::string content;
   size_t position;
   size_t chunk_size;