Constant-time indexing

[samvandevelde]

A pattern I write frequently is the one of constant-time indexing via a linear scan:

 for (index, entry) in array.iter().enumerate() {
    let is_target = index.ct_eq(desired_index);
    value.conditional_assign(entry, is_target);
}

I know that this way of constant-time indexation 'works fine', and if an indexation method would be written it would act the same.
Nevertheless, I wonder why this was omitted from subtle, more specifically whether it was a deliberate design decision.

If no such decisions were taken, if there are no inherent objections to the idea, I additionally ask if a PR for constant-time indexing would be appreciated.

Kind regards,

[tarcieri]

FWIW, ctutils has CtLookup

[samvandevelde]

Sadly, migration from subtle to ctutils is 1) not my call to make and 2) relatively far down the queue.

I see CtLookup follows the same pattern to a tee; all the more justification.

[samvandevelde]

Followed up on the PR: #154