neon-build refactor to make use of dynamic loading

[kjvalencik]

The PR is a little large, but includes several related commits:

• Re-organize neon-build. This commit introduces no functional changes. It re-organizes napi and neon-sys building so they are entirely separate.
• Re-write the napi build to make use of dynamic loading. It only needs to output the index.node file
• Update NAPI tests to use a flattened structure to more accurately reflect the pattern outlined in RFC: Streamlined UX rfcs#36
• Remove the win delay load hook. This is a little sad because it's very neat, but is also no longer necessary. Let me know if you would like to leave it in for future reference.
• Update electron tests to use NAPI. This was important for testing that win delay load hook is no longer necessary.
• Add a neon_build::Builder for customizing the index.node location

[dherman]

Looks great! I just suggest we rename the Builder API.

[dherman]

I think Builder is a confusing name since it doesn't describe what it does -- and it also could be confusing because this crate is neon_build but Builder isn't about running a Neon build.

We talked about Setup::options() being the way to construct this thing. I think it reads nicely.

[dherman]