Add crypto GenerateEntropy() and GenerateHexNonce()

[KendallWeihe]

Closes #51

[mistermoe]

@KendallWeihe thoughts on doing the following for the argument passed to GenerateEntropy and also allowing it to be passed to GenerateNonce?

type EntropySize int

const (
    // Directly set the sizes according to NIST recommendations for entropy
    // defined here: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-90Ar1.pdf
    Entropy112Bits EntropySize = 112 / 8 // 14 bytes
    Entropy128Bits             = 128 / 8 // 16 bytes
    Entropy192Bits             = 192 / 8 // 24 bytes
    Entropy256Bits             = 256 / 8 // 32 bytes
)

this way we can call either function like so:

crypto.GenerateEntropy(Entropy128Bits)
crypto.GenerateNonce(Entropy128Bits)

This provides consumers with easy access to recommended values. wish we could use iota for this but recommended sizes does not increment by a consistent amount that would easily map to iota's default behavior

could also leave off Bits e.g.

const (
    // Directly set the sizes according to NIST recommendations for entropy
    // defined here: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-90Ar1.pdf
    Entropy112 EntropySize = 112 / 8 // 14 bytes
    Entropy128             = 128 / 8 // 16 bytes
    Entropy192             = 192 / 8 // 24 bytes
    Entropy256             = 256 / 8 // 32 bytes
)

[KendallWeihe]

@mistermoe awesome idea! Done ✅ And also added additional test coverage for custom size, and nonce error case, as well as a doc comment for GenerateEntropy()