COMMON: Add WriteStream::writeStream()

[ccawley2011]

No description provided.

[dreammaster]

That could indeed be a nice little helper. A few comments I have:

1. It wouldn't hurt to return the bytes written as a result from the method, just like the write method does
2. It may not hurt to have a second overloaded version of the method that simply takes in a SeekableReadStream and no size, and simply chains to the writeStream method with the stream and stream size. That way, callers that want to copy an entire stream don't have to explicitly specify the size, so long as it's a seekable one (which implements the size() method).

[bgK]

Typically data copying routines use some kind of buffer of a fixed size as temporary storage so that they can work on arbitrarily sized inputs. See what boost does for example:
https://github.com/boostorg/iostreams/blob/develop/include/boost/iostreams/copy.hpp#L123

[bluegr]

This is indeed nice to have, but I believe that it's outside of the scope of this pull request. The code can be adapted later on in-tree

[bluegr]

This looks OK To merge now, thanks! Future enhancements, like @bgK's suggestion for arbitrarily sized inputs, can be done in-tree.