`BoxedUint` support

[tarcieri]

crypto-bigint v0.6.0-pre.0 has a much more featureful BoxedUint type than previous releases which hopefully supports all of the prerequisites required for random prime generation: https://docs.rs/crypto-bigint/0.6.0-pre.0/crypto_bigint/struct.BoxedUint.html

We would like to migrate from num-bigint-dig to crypto-bigint in the rsa crate, notably because the current implementation has a timing sidechannel vulnerability. A full migration will require prime generation / primality testing, hence my interest in this issue.

[fjarri]

We will need something like the traits described in RustCrypto/crypto-bigint#70 (comment) to generalize the methods over Uint/BoxedUint

[fjarri]

I've made a draft PR #36 to see what kind of traits we need for crypto-primes to work