buffer: slow buffer copy compatibility fix

Fix issue where SlowBuffers couldn't be passed as target to Buffer
copy().

Also included checks to see if Argument parameters are defined before
assigning their values. This offered ~3x's performance gain.
buffer: cleanup copy() asserts and tests

Buffer parameter asserts were easily simplified by setting falsy non
numeric-ish values possibly out of bounds to their default value.

Also copy() tests had a flaw that each buffer had the same bit pattern at
the same offset. So even if the copy failed, the bit-by-bit comparison
would have still been true. This was fixed by filling each buffer with a
unique value before copy operations.
buffer: error and misc cleanup

Changed types of errors thrown to be more indicative of what the error
represents. Also removed a few unnecessary uses of the v8 fully
quantified typename.