v8: upgrade to v8 3.20.7
src: update v8 build requirements

In v8 3.20.4 build/common.gypi was moved to build/toolchain.gypi and
build/features.gypi.
v8: reapply floating patches
util: removed duplicated isArray check
stream: Simplify flowing, passive data listening

Closes #5860

In streams2, there is an "old mode" for compatibility.  Once switched
into this mode, there is no going back.

With this change, there is a "flowing mode" and a "paused mode".  If you
add a data listener, then this will start the flow of data.  However,
hitting the `pause()` method will switch *back* into a non-flowing mode,
where the `read()` method will pull data out.

Every time `read()` returns a data chunk, it also emits a `data` event.
In this way, a passive data listener can be added, and the stream passed
off to some other reader, for use with progress bars and the like.

There is no API change beyond this added flexibility.
buffer: slice on zero length buffer

SlowBuffer(0) passes NULL instead of doing malloc(0). So when someone
attempted to SlowBuffer(0).slice(0, 1) an assert would fail in
smalloc::SliceOnto.

It's important that the check go where it is because the resulting
Buffer needs to have external array data allocated. In the case a user
tries to slice a zero length Buffer it will also have NULL passed as the
data argument.

Also fixed where the .parent attribute was set for zero length Buffers.
There is no need to track the source of slice if the slice isn't
actually occurring.
crypto: Add test for sha.write(''); sha.end()
buffer: revert length and parent check

In a rush to implement the fix 35e0d60 I overlooked the logic that
causes 0-length buffer instantiation to automatically not assign the
parent regardless.
buffer: misc logic simplification

Includes:
* No need for `typeof` when checking undefined.
* length is coerced to uint so no need to check if < 0.
* Stay consistent and always throw `new` errors.
* Returning offset + magic number in every write is error prone. Instead
  return the central write function which returns the correct offset.