Support renaming fields in inline records

[tatchi]

Extracted from melange-re/melange#732

[cristianoc]

Thanks a lot for doing this!
Checked, and gentype was already supporting this, so no adaptation required there.

[tatchi]

Btw the original PR also add support for

exception UserException({@as("renamed") name: string})

But this code doesn't parse in rescript (syntax error). Is it something we should support ?

[zth]

@tatchi it's possible this PR might add support for that as a bi effect: #6326

I got stuck and haven't had time to pick that up again. Let me know if that's something you'd be interested in continuing.

[tsnobip]

unfortunately it seems that this PR only works for creating such variants but not for accessing those fields. cf #6438