Remove obj keys mangle

[DZakh]

I've finally found the source of the behavior. Although, I wasn't able to backtrace the reasoning behind it. The code was written 7 years ago by @Hongbo-bb and probably was needed to properly handle @obj or something like this.
But now, I think it only causes wtf moments when the compiled code is not what you expect.

Solves:

• Bug with trying to access an object field starting with underscore #6341
• Object field names with double underscore are compiled incorrectly #6005

Related to:

• Missing mangling props name in JSX v4 #5904
• Make usage of _ in object keys consistent between external and JSX #4348
• Missing mangling props name in JSX v4 #5904

It's a big breaking change, but according to tests it doesn't break anything.

@cristianoc @cknitt @zth @mununki What do you think?

[cristianoc]

Removing it seems the best option for sure.
Though one needs to assess the consequences with a few people trying on their projects.
@cknitt @zth @mununki do you have any code at hand that could be affected?

Also worth checking on the doc website whether there's any mention of this trick.

[cknitt]

@DZakh Great find! 👍

I tested against our current/biggest project (that's already on v11/uncurried) and found no issues with this change.

[mununki]

@DZakh Looking great.

I've tested it in my project and there are no issues, and it doesn't affect the functionality of #5904.

[DZakh]

I didn't expect it, but there's actually a note in the docs https://rescript-lang.org/docs/gentype/latest/usage#renaming-gentypeas-and-object-mangling-convention

[DZakh]

I'll go update changelog 🙏

[cknitt]

@DZakh I noticed that the name mangling is still present in the Gentype code: https://github.com/rescript-lang/rescript-compiler/blob/61c559245dfb43f85a91282f1b775f40138c6ce3/jscomp/gentype/Runtime.ml#L116

[DZakh]

I think we need to remove it from there as well. I'll be able to tackle it around Monday.