Restrict Snowflake range, type changes, consistent naming

[lukellmann]

This PR combines #380, #382 and some additional changes. I would want to keep this PR open until discord/discord-api-docs#3745 gets accepted.

Change some types from String to Snowflake (#380)

• d109712: DiscordGuildBan.guildId
  see:
  https://discord.com/developers/docs/topics/gateway#guild-ban-add-guild-ban-add-event-fields
  https://discord.com/developers/docs/topics/gateway#guild-ban-remove-guild-ban-remove-event-fields
• 9784b5a: DMCreatRequest.userId
  see: https://discord.com/developers/docs/resources/user#create-dm
• 6dfd861: ChannelFollowRequest.webhookChannelId
  see: https://discord.com/developers/docs/resources/channel#follow-news-channel

Restrict Snowflakes to valid range (#382)

Implement the Snowflake limitations described in discord/discord-api-docs#3745. This will e.g. fix an error in pagination (invalid Snowflakes were used before).

The valid range for a Snowflake's raw value is represented by the new validValues property of Snowflake's companion object. All constructors will coerce their input in that range so there is no way to create an invalid Snowflake.
If Discord ever decides to widen the valid range, it would just be a matter of adjusting validValues accordingly to support that. (I intended this range to be the single point of truth).

b96f3bb removes a test that was depending on an invalid Snowflake to pass. Since there is no longer a way to create such invalid Snowflakes, it is impossible to make this test pass.

Consistent naming

• 957fcc4 renames Snowflake.discordEpochStart to Snowflake.discordEpoch as it is the term used in Discord's documentation
• fd4f387 brings consistent naming to all XxxTimestamp classes and properties