Update syntax for Solidity 0.6.0

[haltman-at]

Welp, it seems Solidity 0.6.0 is here sooner than I expected! Fortunately it didn't include as many changes as originally planned...

I've updated the syntax in the following ways:

• Added the new keywords abstract, virtual, override, fallback, receive, try, and catch
• Added the new assembly keyword leave
• Added address to the list of generic members? Not sure how that'll play out exactly but added it FWIW
• Added Error as a builtin; not sure that's exactly the right category for it but I thought it made sense to add and that seemed like the closest thing
• Altered the regular expressions for hex strings so that they are now required to be of even length, and to allow an optional single underscore between each byte

[haltman-at]

Good idea, added those comments!