Bug fix. Changed += to = in FieldVar double_in_place

[mskrzypkows]

Description

Fixed bug in Field_var::double_in_place

closes: #XXXX

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[mmagician]

Wasn't there a unit test for this before? Could you add one for future proofing?

[mskrzypkows]

I don't see any tests for FieldVar. I'll add some.

[mskrzypkows]

@mmagician What do you think, can we merge it?

[mmagician]

It's currently failing due to recently introduced breaking changes in nightly. Unfortunately there's no automatic fix for these (yet, see WIP), so removing the redundant imports needs to be done manually for now.

[mskrzypkows]

@mmagician, @Pratyush removed unused imports, should pass now.

[mskrzypkows]

🤔 Something changed in algebra with ark_std::sync?

[mskrzypkows]

Can we rerun failing no_std check?