Create a HandleScope before calling the Environment::GetCurrent() that
takes a v8::Isolate* as an argument because it creates a handle with
the call to v8::Isolate::CurrentContext().

Because it's possible for the data within a Buffer instance to be
altered after instantiation, or in case a user attempts to do something
like the following:

Buffer.prototype.fill.call({}, 10, 0, 10);

It doesn't result in a segfault.

This commit makes it possible to use multiple V8 execution contexts
within a single event loop.  Put another way, handle and request wrap
objects now "remember" the context they belong to and switch back to
that context when the time comes to call into JS land.

This could have been done in a quick and hacky way by calling
v8::Object::GetCreationContext() on the wrap object right before
making a callback but that leaves a fairly wide margin for bugs.

Instead, we make the context explicit through a new Environment class
that encapsulates everything (or almost everything) that belongs to
the context.  Variables that used to be a static or a global are now
members of the aforementioned class.  An additional benefit is that
this approach should make it relatively straightforward to add full
isolate support in due course.

There is no JavaScript API yet but that will be added in the near
future.

This work was graciously sponsored by GitHub, Inc.

Due to a lot of the util.is* checks there was much unnecessary overhead
for the most common use case of Buffer. Which is creating a new Buffer
instance for data from incoming I/O. NativeBuffer is a simple way to
bypass all the unneeded checks and simply hand back a Buffer instance
while setting the length.

This and b80d11d are my fault. An unrelated test was failing, which
prevents {js,cpp}lint from running automatically.

For several encodings the byte length is simple arithmetic. Don't call
into C++ in those cases.

* Change calls to String::New() and String::NewSymbol() to their
  respective one-byte, two-byte and UTF-8 counterparts.

* Add a FIXED_ONE_BYTE_STRING macro that takes a string literal and
  turns it into a v8::Local<v8::String>.

* Add helper functions that make v8::String::NewFromOneByte() easier to
  work with. Said function expects a `const uint8_t*` but almost every
  call site deals with `const char*` or `const unsigned char*`. Helps
  us avoid doing reinterpret_casts all over the place.

* Code that handles file system paths keeps using UTF-8 for backwards
  compatibility reasons. At least now the use of UTF-8 is explicit.

* Remove v8::String::NewSymbol() entirely. Almost all call sites were
  effectively minor de-optimizations. If you create a string only once,
  there is no point in making it a symbol. If you are create the same
  string repeatedly, it should probably be cached in a persistent
  handle.