Replace rug crate with num-bigint

[ljoy913]

This PR replaces the use of the rug crate in forks/Spartan-t256/circ_fields with num-bigint to remove the dependency on gmp-mpfr-sys.

The rug crate is a wrapper around gmp-mpfr-sys, which in turn wraps the GNU Multiple Precision Arithmetic Library (GMP). While GMP is a high-performance C library for arbitrary-precision arithmetic, it includes CPU-specific assembly code that:

• Cannot be cleanly compiled to WebAssembly
• Fails to build on Windows with the x86_64-pc-windows-msvc toolchain due to its reliance on a GNU toolchain during compilation

Changes

• Replaced rug with num-bigint throughout circ_fields
• Updated the num-bigint dependency from =0.4.3 to ^0.4 to allow the latest compatible 0.4.x version to be used
  • The minimum required version is 0.4.5, which introduces the modinverse method needed to match functionality previously provided by rug
  • The current latest version is 0.4.6
• Added a couple of functions to replace rug functionally not present in num-bigint

[ljoy913]

No longer necessary.