buffer: check logic simplification

Checks have been simplified and optimized for most-used cases.

Calling Buffer with another Buffer as the subject will now use the
SlowBuffer Copy method instead of the for loop.

No need to call for value coercion, just place the ternary inline.
deps: upgrade libuv to 26fa6f8
stream_wrap: remove superfluous buffer len check

It's a buffer so it's never bigger than Buffer::kMaxLength bytes,
which in turn is always < INT_MAX.
path: Do not coerce paths to strings on Windows

Fix #4795