[Ur] Change grammar to include fewer keywords

[Adam Chlipala]

There's an open GitHub issue <https://github.com/urweb/urweb/issues/204> 
suggesting that the language definition include fewer keywords.  That 
way, the identifiers [Type], [Name], [Unit], and [map] become available 
for user-defined identifiers (as opposed to somewhat unappetizing 
alternatives like [mp] and [Nam] that I've gotten used to).

I think I'm pretty convinced now that the associated PRs make sense, but 
I figured I'd run the idea past the community here before committing.  
Please let me know if you have any objection or suggestion for a 
different code change.

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