New Buffer API backport to v4.x

[ChALkeR]

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

buffer

Description of change

This backports the new Buffer.alloc(), Buffer.allocUnsafe(), Buffer.from(), and Buffer.allocUnsafeSlow() APIs for v4.

Note: some backported tests are disabled, but those are not related to the new API.

Important: Buffer.from(arrayBuffer[, byteOffset [, length]]) (as in 6.x) was not backported, I used Buffer.from(arrayBuffer) instead, that helped to significantly reduce the code changes.

Note that --zero-fill-buffers is not included here (see #5745 for that), also the encoding parameter for .fill() is not included.

Existing code changes are minor here, this is not invasive.

Only two changes in existing code:

1. .fill('') now zero-fills,

2. One call to Buffer(string, encoding) is replaced with Buffer.from.

   There is a typecheck in that code path, so the behaviour is identical. In fact it could be reverted back to Buffer(string, encoding) or even changed to fromString(string, encoding) — that would just affect the double checks.

All the other changes are additive, i.e. introduce new methods and don't change the existing ones.

I'm hoping that this and #5745 will make it into 4.5.0.

/cc @jasnell @nodejs/ctc @seishun