fs: pool size coincide with ReadStream bufferSize

pool size of file reading in ReadableStream can be adjustable
with a bufferSize option.
fs: remove unused default option in WriteStream

No bufferSize option is used in stream.Writable and fs.WriteStream.
src: use static_cast where appropriate

Use static_cast instead of reinterpret_cast when casting from void*
to another type.

This is mostly an aesthetic change but may help catch bugs when the
affected code is modified.
dtrace: x64 ustack helper