PR checklist after recent (2023-08-01) sync with upstream libsecp256k1

[jonasnick]

After updating this repository with over a year worth of commits from libsecp256k1, some updates to open PRs may be necessary.

• Make position of * consistent in -zkp include/ files (void *foo not void* foo)
• Don't use SECP256K1_CONTEXT_VERIFY or SECP256K1_CONTEXT_SIGN flags. They have been deprecated. There are only two different contexts now: 1. A context initialized with SECP256K1_CONTEXT_NONE 2. Static contexts, which do not have the ecmult_gen context built.
• Get rid of "initialized for signing" terminology in include/ files. Instead use something like

  *  Args:    ctx:       pointer to a context object (not secp256k1_context_static).
  

• Use global CTX or STATIC_CTX in tests
• Rename secp256k1_context_no_precomp to secp256k1_context_static