smalloc: allow different external array types

smalloc.alloc now accepts an optional third argument which allows
specifying the type of array that should be allocated. All available
types are now located on smalloc.Types.
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.