Update syntax for Solidity 0.8.4

[haltman-at]

Syntax updates for Solidity 0.8.4:

1. I've promoted revert from a builtin to a keyword. Really it's both now, but I figure keyword wins?
2. I've added error as a keyword.
3. I've added the new builtin bytes.concat.
4. I was going to add hex strings in assembly, but it turns out we actually already had those even though they apparently didn't exist previously! :P

[joshgoebel]

Reminder: lexemes has been deprecated for quite a while and soon to be removed with v11. Use keywords.$pattern instead.

[haltman-at]

I was unaware of that, thanks. So I guess we'll have to change that soon. But I'm going to skip doing that on this PR since this is just a quick syntax update that I also ideally want to get out quickly, and we can handle that problem together with the other transition stuff either in #37 or a separate PR?

[joshgoebel]

It was just a heads up, wasn't expecting you to merge it with this work.

[frangio]

LGTM!