feat(algebraic_geometry): structure sheaf on Spec R (#3907)
This defines the structure sheaf on Spec R, following Hartshorne, as a sheaf in `CommRing` on `prime_spectrum R`.
We still need to show at the stalk at a point is just the localization; this is another page of Hartshorne, and will come in a subsequent PR.
Co-authored-by: Kevin Buzzard <k.buzzard@imperial.ac.uk>
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Ashvni <ashvni.n@gmail.com>
