Add const_iterable_sequence concept

[tcbrindle]

For a sequence S, const_iterable_sequence<S> means we can use S const as a sequence with the expected semantics.

Specifically, we require that:

• S const is a sequence
• S and S const have the same cursor type and the same value type
• S and S const have the same const_element_type
• For every extended sequence concept (multipass, bidirectional etc), excluding read_only_sequence, both S and S const must satisfy the concept or both must not satisfy the concept. For example, if S is bidirectional then S const must be bidirectional and vice versa
• If S models read_only_sequence then S const must model read_only_sequence -- that is, S const may strengthen the const-qualification of elements but not weaken it
• Semantic requirement: iterating over S const yields "the same elements" as iterating over S (but let's gloss over exactly how we define "same"...)

We've previously assumed all of these things are true (except possibly the const_element_t requirement), but it's good to have them checked by the compiler where we can.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b54eb3f) 97.99% compared to head (b9fb820) 97.99%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #154 +/- ## ======================================= Coverage 97.99% 97.99% ======================================= Files 66 66 Lines 2392 2392 ======================================= Hits 2344 2344 Misses 48 48 ```

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