Scaling bottlenecks in the free-threaded build

[colesbury]

There are a few remaining scaling bottlenecks in the free-threaded build that we should fix.

I have been using the following benchmark to detect bottlenecks that were previously issues in older versions of the nogil forks:
https://gist.github.com/colesbury/429fe9f90036d43ad43576c3d357a12e

Note that for reliable results the above benchmark requires some setup:

• Adjust NTHREADS if necessary on your system
• Disable turbo boost or equivalent on your system
• Avoid running on hyper-threading siblings (i.e., use taskset -c 0-<N> to choose separate physical cores)

Current bottlenecks

• cmodule_function
• load_string_const
• load_tuple_const
• create_closure

Underlying issues

• Reference count contention on non-string constants. We will want to immortalize most constants in PyCodeObject.
• Reference count contention on func.__qualname__ or code.co_qualname (when creating closure)
• Reference count contention on module-level PyCFunctionObjects

Linked PRs

• gh-118527: Use _Py_ID(__main__) for main module name #118528
• gh-118527: Use deferred reference counting for C functions on modules #118529
• gh-118527: Intern filename, name, and qualname in code objects. #118558
• gh-118527: Intern code name and filename on default build #118576
• gh-118527: Intern code consts in free-threaded build #118667