Add convert_let_attrset assist

[MoritzBoehme]

I'm not very happy with how the choice between body and bindings works.
If anyone is reading this and has some spare time, feel free to suggest better ways to do this.

[oxalica]

Is this really useful? I have not seen any let {} in wild besides accidental usages like typos. So only a warning is enough to me.

Also note that let { ... } desugars to rec { ... }.body, all fields including body can mutual reference, and body is accessed at eval-time. Your conversion doesn't preserve semantics in all cases.

let { a = [ body ]; body = [ a ]; } # Valid and evaluates to an infinite list.

let a = [ body ]; in [ a ] # Syntactically invalid. (Parse failed for Nix)

let a = [ body ]; body = [ a ]; in body # Valid. Same result.

let { } # Syntactically valid but error when evaluated.

let in body # Syntactically invalid. (Parse failed for Nix)

[MoritzBoehme]

Is this really useful?

Probably not. I had to look up how the old let worked and only found 2 examples in total.