Refactor zero-copy buffers for performance and to prevent memory leaks

* In order to prevent ArrayBuffers from getting garbage collected by V8,
  we used to store a v8::Persistent<ArrayBuffer> in a map. This patch
  introduces a custom ArrayBuffer allocator which doesn't use Persistent
  handles, but instead stores a pointer to the actual ArrayBuffer data
  alongside with a reference count. Since creating Persistent handles
  has quite a bit of overhead, this change significantly increases
  performance. Various HTTP server benchmarks report about 5-10% more
  requests per second than before.

* Previously the Persistent handle that prevented garbage collection had
  to be released manually, and this wasn't always done, which was
  causing memory leaks. This has been resolved by introducing a new
  `PinnedBuf` type in both Rust and C++ that automatically re-enables
  garbage collection when it goes out of scope.

* Zero-copy buffers are now correctly wrapped in an Option if there is a
  possibility that they're not present. This clears up a correctness
  issue where we were creating zero-length slices from a null pointer,
  which is against the rules.

1 files changed, 1 insertions, 1 deletions

diff --git a/core/libdeno/test.h b/core/libdeno/test.h
index dd5dc99b2..d7e6a3f80 100644
--- a/core/libdeno/test.h
+++ b/core/libdeno/test.h
@@ -6,7 +6,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 extern deno_snapshot snapshot;  // Loaded in libdeno/test.cc
-const deno_buf empty = {nullptr, 0, nullptr, 0, 0};
+const deno_buf empty = {nullptr, 0, nullptr, 0};
 const deno_snapshot empty_snapshot = {nullptr, 0};
 
 #endif  // TEST_H_