[Ur] Change grammar to swap precedence of "&&" and "||"?

[Adam Chlipala]

Way back when, in Ur/Web's grammar, I apparently gave these two 
quintessential Boolean operators different relative precedences, 
compared to other widely used languages, as has been pointed out 
<https://github.com/urweb/urweb/issues/202>. Would anyone object to 
swapping them?  It's entirely possible that there has not yet been 
written an Ur/Web program that would parse differently depending on this 
choice, but it seems worth checking for strong opinions.  A week from 
posting this message, I'll consider it safe to take the proposed action, 
if no one has complained.

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