Implement Polynomial_real_arb

[user202729]

Mostly copy-paste from Polynomial_complex_arb with acb replaced with arb.

For ease of reviewing, the change is split into two commits, the first one is copy-paste (thus can be easily diffed against Polynomial_complex_arb), and the second one is the change on top of that.

Results in a large speedup (especially for multiplication of polynomials with coefficients all having similar magnitudes, since flint use FFT when appropriate with $O(n \log n)$ algorithm, while Sage generic implementation mostly use schoolbook).

The documentation doesn't show up on the website (HTML), but polynomial_complex_arb doesn't show up either, so this is mostly out of scope. To make it show up, one need appropriate .rst file in src/doc/en/reference (they were originally generated with sphinx-autodoc, I think)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

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