[Implement] isBuffer<T>(value: T) - #8

@dcodeIO

> Buffer.isBuffer(null)
false

We need to do a null check, right?

The instanceof operator should handle this if T is nullable.

Alright. Then this should be all set

Given that it's possible for T to be nullable, maybe this check is still okay? Unless if T is nullable it generates a null check too.

If T is statically nullable, the instanceof will implicitly test for null.

In that case we should be good. Will merge later.