Implement FromStr for Snowflake-related types

[ext4cats]

As of v0.10.10 it seems all the Snowflake types (GuildId, ChannelId, etc.) are missing an implementation of the FromStr type. The BSON format, for instance, does not support unsigned 64 bit integers, which means the only alternative is to serialize and deserialize them as strings. However, this can't be done trivially via serde_with::DisplayFromStr because the aforementioned types do not implement the FromStr trait, needed for deserialization.

[nickelc]

The ID types in the next branch are now serialized as string.

serenity/src/model/id.rs

Lines 259 to 273 in 0391d7c

	pub(crate) mod snowflake {
	use std::convert::TryFrom;
	use std::fmt;
	use serde::de::{Error, Visitor};
	use serde::{Deserializer, Serializer};
	pub fn deserialize<'de, D: Deserializer<'de>>(deserializer: D) -> Result<u64, D::Error> {
	deserializer.deserialize_any(SnowflakeVisitor)
	}
	#[allow(clippy::trivially_copy_pass_by_ref)]
	pub fn serialize<S: Serializer>(id: &u64, serializer: S) -> Result<S::Ok, S::Error> {
	serializer.collect_str(id)
	}