node 10 support (part 1)

[deancouch]

• use the new Buffer constructors Buffer.alloc and Buffer.from
• fix the range of values generated for the writexxx tests to now better match the signed / unsigned functions
• copy the node 8 functions (https://github.com/nodejs/node/blob/v8.x/lib/buffer.js) into the reader / writers and use these (note we may need to include the copyright notice)

This allows us to release a non-breaking change that is compatible with node 8 and node 10.

part 2 will be a breaking change and include:

• switching back to the real node Buffer.writexxx / Buffer.readxxx functions
• remove the noAssert functionality
• bump ref to 1.3.5