Add PrimeField::from_bytes_mod_order

[ValarDragon]

Description

Adds a method from_bytes_mod_order, needed for hashing to the curve (#147).

This also includes a python file I used for creating test vectors for the method. I don't know how to write useful tests for this function for a generic field (since we'd need to do arithmetic on integers of a length longer than the field). Hence this function is only tested on one field, but includes a script to add test vectors for any field.

ref: zkcrypto/ff#45
closes: #156

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• Targeted PR against correct branch (main)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
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[weikengchen]

The PR looks good. Especially the test is comprehensive.

[Pratyush]

I don't know how to write useful tests for this function for a generic field

We have access to the Size of the modulus generically, so we can use that to generate enough bytes, right? And for the independent source of truth we can use something like num-bigint as a dev-dependency

[ValarDragon]

Didn't realize there were pre-existing variable sized big-int libraries. Do you think we should go with a general approach for testing before merging this, or have a todo for using num-bigint as a dev-dependency?

[weikengchen]

I think it would be fine to use num-bigint. Indeed, nonnative already uses it (https://github.com/arkworks-rs/nonnative/blob/master/Cargo.toml)

[ValarDragon]

Switched to using num-bigint for checking this. Is it fine to keep it with the test values being generated via python?
(So the test works for any fields, but the values tested on aren't particularly interesting for modulii of different byte lengths)

[Pratyush]

Suggested change

	for i in num_bytes_to_directly_convert..bytes.len() {
	res *= window_size;
	res += Self::from(bytes[i]);
	}
	for byte in bytes[num_bytes_to_directly_convert..] {
	res *= window_size;
	res += Self::from(byte);
	}

[Pratyush]

Should we also add tests for le?

[Pratyush]

btw for the test, does it also make sense to test with random inputs?

[ValarDragon]

Now that we have a reference, it makes sense

[Pratyush]

This LGTM modulo the nit and the comment about tests.