Improvements to hypergeometric motives

[kedlaya]

In the wake of #23671, a meta-ticket to record some proposed improvements to hypergeometric motives. Please add suggestions to this list, and make individual tickets for individual changes.

• Euler factors at tame and wild primes.
• Optimizations to speed up padic_H_value. Some occur at #24433 (cache coefficients of the Dwork exponential series) and #28458 (eliminate redundant Gauss sum calls).
• Recode the computation of Gauss sums and/or the hypergeometric trace formula in Cython.
• Speed up possible_hypergeometric_data for fixed weight. (See #23953 for one step: returning an iterator instead of a list.)
• Add the computation of the Frobenius structure on the hypergeometric equation, which can be more efficient than the trace formula when the degree is large.
• Refactor: should this really be under modular?