[m-rev.] for review: add <<u and >>u as operators

[Julien Fischer]

On Sun, 4 Dec 2022, Zoltan Somogyi wrote:

> Recognize <<u and >>u as operators.
> 
> library/ops.m:
>     Add these to the list of operators, at the same priority as << and >>.
> 
> library/mercury_term_lexer.m:
>     <<u and >>u are the first operator that combine graphic characters with

s/operator/operators/

>     alphanumeric characters. This requires extending the lexer, because
>     if the lexer, when given e.g. the string "<<u", returns "<<" and "u"
>     as separate tokens, then the presence of <<u in the ops table
>     would not matter.
>
>     Implement this special case in all three different variants of the lexer.
> 
> doc/reference_manual.texi:
>     Document the new operators, and their special-case nature.

That's fine otherwise.

Julien.