Handle base conversion nicer

[ChihChengLiang]

Problem

The current codebase uses many patterns that are difficult to reason and debug.

For example,

pattern 1

b13_chunks.iter().fold(0, |acc, x| { acc * B9 + convert_b13_coef(*x)})

This requires b13_chunks to be in big endianness, but a reader might not be aware of it.

Solution: We collect them in the same function f_from_radix_be so that we have to reason only once. And from the function name, we know the input must be in big-endian.

pattern 2

We do a lot of manual base conversion like

    for _ in 0..64 {
        let remainder: u64 = mod_u64(&raw, B9);
        acc += coef_transform(remainder) * base.clone();
        raw /= from_base;
        base *= to_base;
    }

Solution: The bigUint has an expressive to_radix_be API to allow us to perform base conversion easily.

[CPerezz]

LGTM!

Thanks for this. As we discussed will make us easier to debug the error in #268