Message embeds

[Drahcirius]

Branch history continuing from #1345

message.channel.fetchMessage("318040520050081793").then(m => {
  const embed = m.embeds[0];
  embed.title = "New Title";
  m.edit({ embed });
});

The code above would give a Converting circular structure to JSON error when sending a changed message embed object. This patch allows for message embeds to be re-sent without getting these errors.

A more complete version of #1452 was also included in this patch, which makes this a breaking change because it removes the reference from the embed to the original message. This makes doing message.embeds[0].message no longer possible.

Credits to @devsnek for writing the initial embed transform code.

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.