Refactor UInt{8,16,64,128} into one struct UInt

[Pratyush]

Description

closes: #116

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[mmagician]

Looks good so far.
I wonder whether we could completely get rid of uint8 module?
To preserve+ the functionality, it would require the UInt<const N: usize, T: PrimInt + Debug, F: Field> struct to implement new_input_vec, which would in turn require an extra trait bound T: ToConstraintField. These seem like natural things to have for u16 etc. anyway.

This could easily be a follow-up PR though, but if you agree then let's open an issue for tracking.

[Pratyush]

Unfortunately I think there's a special impl of ToConstraintField for [u8]s that's used in the new_input_vec impl. Generalizing it to support larger sizes shouldn't be difficult, but would require moving away from calling ToConstraintField. Maybe that's fine though.

[mmagician]

Rather than generalizing we could just implement ToConstraintField for &[u16] etc no?

[autquis]

@Pratyush After moving ToBytesGadget to convert, it is no longer part of prelude. Is it intended?
I'm fixing crypto-primitive for this change. For example, in:
https://github.com/arkworks-rs/crypto-primitives/blob/8bbb4824ede5f2cf115f1a37e08410b57944a4f6/src/signature/constraints.rs#L10

It was previously enough to use ark-r1cs-std::prelude::* but now I need either use it explicitly or update the prelude.