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:

• List API is inconsistent with other modules rescript-core#192
• Use optional arguments in bindings rescript-core#206
• Docstrings missing for some modules
• Migration from Js/Core to Stdlib - see [WIP] Migrate command #7693
• 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