Include higher-order functions in --dump-stan-math-signatures

[andrjohns]

Submission Checklist

• Run unit tests
• Documentation
  • [] If a user-facing facing change was made, the documentation PR is here:
  • [] OR, no user-facing changes were made

Updates the pretty-printing for the math signatures to include all higher-order functions, using ... for variadic functions. For example:

dae((real, vector, vector, ...) => vector, vector, vector, real, array[] real, ...) => array[] vector
laplace_latent_bernoulli_logit_rng(array[] int, array[] int, vector, int, function(...) => matrix, tuple(...)) => vector
reduce_sum(function(array[] real, data int, data int, ...) => real, array[] real, int, ...) => real
reduce_sum(function(array[,,,,,,] row_vector, data int, data int, ...) => real, array[,,,,,,] row_vector, int, ...) => real

Release notes

All higher-order function signatures now printed in --dump-stan-math-signatures

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[andrjohns]

Ah the expression tests. I'll open a Math PR to conditionally add expression tests for the new signatures

[WardBrian]

For the few existing higher order functions that already get printed, we don't print the word function, e.g.
https://github.com/stan-dev/stanc3/blob/8e154ac3454790cb5427ae72106c859b3fb8d90a/test/integration/signatures/stan_math_signatures.t#L12745

I'm mostly ambivalent about what we do print, but it should be consistent.