Consolidate Stdlib APIs before v12 release

[cknitt]

Creating this issue here for tracking as https://github.com/rescript-association/rescript-core is in a different org.

Before v12 release, the APIs of the new Core standard library that has just been merged into the compiler repo should be as stable as possible.

Known issues:

• [x] https://github.com/rescript-association/rescript-core/issues/192
• [x] https://github.com/rescript-association/rescript-core/issues/206
• [x] Docstrings missing for some modules
• [ ] Comby migration script outdated/incomplete
• [ ] Make sure to use Stdlib instead of Core everywhere (e.g. also tests/tests/src/stdlib instead of tests/tests/src/core)

[fhammerschmidt]

Still some more docstrings missing, but that is tracked in another issue: #7546