chore: remove temporary allocations from num_bits

[TomAFrench]

Description

Problem*

Closes #4759

Summary*

I found this due to #6550.

We make heavy use of the num_bits method during compilation but this is currently doing a lot of unnecessary allocations in order to perform a full bit decomposition of the field element. We can rework this to work on the byte decomposition which removes 96% of all the temporary allocations from compilation.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.