Support mutable for nested record fields

[DZakh]

This is a really nice feature and it would be amazing to be able to use mutable together with it 🚀

[zth]

@DZakh odd that it's not already possible. Should be a very simple parser error. There's nothing technically stopping this from working, so it's probably just some oversight in how the fields are parsed when nested.

[cknitt]

@shulhi is this something you could have a look at?

[zth]

@shulhi it should be some simple mistake in the setup. All the parts should be there in terms of the parsing, just some mistake somewhere.

[shulhi]

Yeah, I can take a look.