bug#37849: composable character alternatives in rx

[Mattias Engdegård]

This patch adds `union' and `intersection' to rx. They both take zero or more 
charsets as arguments. A charset is either an `any' form that does not contain 
character classes, a `union' or `intersection' form, or a `not' form with 
charset argument.

Example:

(rx (union (any "a-f") (any "b-m")))
=> "[a-m]"

(rx (intersection (any "a-f") (any "b-m")))
=> "[b-f]"

The character class limitation stems from the inability to complement or 
intersect classes in general. It would be possible to partially lift this 
restriction for `union'; it is clear that

(rx (union (any "ab" space) (any "bc" space digit)))
=> "[abc[:space:][:digit:]]"

but it makes the facility harder to explain to the user in a way that makes 
sense. Still, it could be a future extension.

A `difference' operator was not included but could be added; it is trivially 
defined in rx as

(rx-define difference (a b)
  (intersection a (not b)))

The names `union' and `intersection' are verbose, but should be rare enough 
that it's better with something descriptive.
SRE, from where the concept was taken, uses `|' and `&' respectively, and `~' 
for complement, `-' for difference.

0001-Add-union-and-intersection-to-rx-bug-37849.patch
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