re-factor Buffer to work more like regular Node

[gbrail]

We should rely more on the standard "buffer.js" code from Node and less on custom Java code. That way it is less work to stay compatible. That means implementing "SlowBuffer" and "Buffer" just like Node 10.

[gbrail]

The solution to this problem was to continue to manage the guts of the buffer ourselves, but to use the function prototypes from the buffer.js file, with some changes so that we could do this. The result is better compatibility with all of the little checks and things that the tests require.