feat: serialize XorName to hex string

[b-zee]

A problem arises when one wants to convert a structure like HashMap<XorName, usize> into a human-readable format like JSON. These formats do not allow complex types like XorName to be used as a key in a key-value structure, but instead only allow strings, for example.

This PR adds a feature that serializes XorName and Prefix into a hex string only if serializing into human-readable format.

I have added the following to the README.md:

Serialization

XorName and Prefix can be serialized into a human-readable hex string, instead of as a u8 array. To enable this, activate the serialize-hex feature. This also allows for these structures to be serialised when used as a key in a map like HashMap, because most formats only allow keys to be strings, instead of more complex types.

A struct like this:

#[derive(Serialize, Deserialize)]
struct MyStruct {
    prefix: Prefix,
    xor_name: XorName,
}

Will yield this JSON

{
  "prefix": "8a817b6d791f4b00000000000000000000000000000000000000000000000000/56",
  "xor_name": "8a817b6d791f4bae4117ac7ae15a88cd2c62fba0b040972ce885f1a47625dea1"
}

instead of

{
    "prefix": {
        "bit_count": 56,
        "name": [141,199,202,57,183,222,153,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
    },
    "xor_name": [141,199,202,57,183,222,153,14,185,67,253,100,133,71,118,221,133,170,130,195,58,66,105,105,60,87,179,110,7,73,237,143]
}

[davidrusu]

nice

[davidrusu]

bors r+