buffer: add NativeBuffer API

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.
buffer: fix assert fail from JS API

Length arguments passed to SlowBuffer were coerced to Int32, not Uint32,
so passing a negative number would throw the following:

node: ../src/smalloc.cc:244: void node::smalloc::Alloc(): Assertion `length <= kMaxLength' failed.
Aborted (core dumped)

That has been fixed by coercing to Uint32 and comparing the value
against kMaxLength.