Improve Safe Promises with ideas from rescript-promise

[cristianoc]

Safe promises are introduced here: #5709
The PR changes as little as possible, just covers the safety aspect.

There are other, orthogonal improvements, performed here:
https://github.com/ryyppy/rescript-promise
these should be integrated as well.

There are a few open questions:

• Should https://github.com/ryyppy/rescript-promise also be made safe?
• Also, the docs of https://github.com/ryyppy/rescript-promise are written with the idea that direct promise access would be the main mechanism. With async/await we just don't know what the main mechanism will be.

CC @ryyppy

[cristianoc]

This goes elsewhere.