http: Use streams3 directly, not .ondata/end
stream: Short-circuit buffer pushes when flowing

When a stream is flowing, and not in the middle of a sync read, and
the read buffer currently has a length of 0, we can just emit a 'data'
event rather than push it onto the array, emit 'readable', and then
automatically call read().

As it happens, this is quite a frequent occurrence!  Making this change
brings the HTTP benchmarks back into a good place after the removal of
the .ondata/.onend socket kludge methods.
src: use v8::String::NewFrom*() functions

* 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.