buffer: add encoding parameter to fill()

Can now call fill() using following parameters if value is a String:

    fill(string[, start[, end]][, encoding])

And with the following if value is a Buffer:

    fill(buffer[, start[, end]])

The encoding is ignored if value is not a String. All other non-Buffer
values are coerced to a uint32.

A multibyte strings will simply be copied into the Buffer until the
number of bytes run out. Meaning partial strings can be left behind:

    Buffer(3).fill('\u0222');
    // returns: <Buffer c8 a2 c8>

In some encoding cases, such as 'hex', fill() will throw if the input
string is not valid.

PR-URL: https://github.com/nodejs/node/pull/4935


Reviewed-By: James M Snell <jasnell@gmail.com>