Rename some keyword locations for consistency

[Earlopain]

Prism is generally good about their naming but these had some inconsistencies compared to others:

• InNode was missing the keyword in the name
• MatchPredicateNode in was named operator instead
• UnlessNode/UntilNode/WhenNode/WhileNode has more than one keyword, with one generic keyword_loc
• UntilNode/WhileNode named their end keyword closing_loc

Left the old names around for compatibility

[kddnewton]

I'm generally okay with doing this, but I just want to check — can you enumerate our rules for naming? Maybe we should explicitly write them down? Is it keyword_loc if there's only one and then *_keyword_loc when there are multiple?

[Earlopain]

Yeah, you got that right basically.

Some like {/do for blocks are just opening since it's not exclusively for keywords.

IfNode is a bit messy since it's for both keyword and ternary and it reuses the keyword locations for :/? with ElseNode. Conceptually they are very close but structure is pretty different. Seems like that should maybe have been a separate node but I don't want to touch that.

I don't really know what where to put docs about general rules, is there a place that would fit? I can write something if you want (although I don't think it would be relevant very often, should just look at rdoc)