Cleanup modules for mathematical functions

[fredrik-johansson]

The following modules are too narrow in scope to exist as standalone modules:

• partitions
• bernoulli

At the same time, there are too many special-purpose functions in the arb module.

Idea: maybe create a dedicated module for combinatorial/number-theoretic functions. This could host partitions, bernoulli and functions like bell, fib currently located in the arb module. What would be a good name for such a module?

• arb_combinat? (...but the functions are not strictly combinatorial)
• arb_arith / arb_arithfun? (...but the functions are not strictly arithmetical)
• arb_sequences? (...but some functions don't naturally correspond to linear sequences)
• arb_integers? (...but some functions return rational numbers)

It's a bit annoying to give all functions prefixes based on the module, but maybe this convention could be relaxed. We might want to do this elsewhere, e.g. provide arb_erf as an alias for arb_hypgeom_erf.

Related:

• hypgeom could just be merged with arb_hypgeom.
• There have also been complaints about about dirichlet.h being a too generic name.