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What is the new behavior?

to_bits_le and to_bits_be implementations are modified to take into account existing Lambdaworks implementation: https://github.com/lambdaclass/lambdaworks/blob/1a6d14ee7def60d0692d2b8fd323ca0d494920fd/math/src/field/fields/fft_friendly/stark_252_prime_field.rs#L50-L67
Unit tests are modified accordingly
bitvec dependency is removed