Three minor rx additions follow:

* Add `anychar' as an alias for `anything': the latter suggests an expression that can match any string, while in reality it only matches a single character. The documentation now uses `anychar' as the preferred name. (`any-char' would also be possible, but is longer.)

* Add `unmatchable' for a never-match regexp. This follows the previously introduced variable `regexp-unmatchable'.

* Add `unordered-or' as a variant of `or' without the left-to-right match order guarantee. It allows unconditional regexp-opt optimisations, and is particularly useful for matching sets of keywords. With rx-let and rx-define, it also has the potential for better compositionality, allowing expressions to be put together from smaller parts.

Abstractly: while `or' is associative, `unordered-or' is also commutative.

The name `unordered-or' is descriptive but phonetically (and lexically) somewhat weak. Strong alternatives welcome.