Use <ostream> and <istream> narrower includes, not <iostream>

[jwnimmer-tri]

🦟 Bug fix

Summary

The use of <iostream> induces global constructors in every translation unit to initialize std::cout and std::cerr. We shouldn't do that unless we're actually going to use std::cout or std::cerr.

Reference: https://en.cppreference.com/w/cpp/header/iostream

Checklist

• Signed all commits for DCO
• Added tests (N/A)
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing) -- no new errors; the 9 errors on the main branch are also reported on this PR.
• All tests passed (See test coverage)
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[chapulina]

Thanks!