Provide reasonable default definition for is_contiguous

[user202729]

Another attempt at #4716 . Compiling base-test.cc takes around 10 seconds (both before and after), probably is fine. (not sure what else to measure.)

Instead of using contiguous_iterator, I just check whether .data() can be called on it; if it can be called, the container is likely contiguous. Opinion?

Some code assumes that if it's contiguous, .resize() exists, so it won't work for e.g. std::array even though that one is contiguous. But then back_insert_iterator doesn't work on std::array either.

Directly define a partial specialization of is_contiguous with typename = void leads to ambiguous overload errors, which is backwards incompatible if someone else special-case is_contiguous. See for example https://github.com/user202729/fmt/actions/runs/24015836992/job/70035092887