[WIP] bpo-39984: Add PyInterpreterState.ceval

[vstinner]

subinterpreters: Move _PyRuntimeState.ceval.recursion_limit to
PyInterpreterState.ceval.recursion_limit: make the limit
per-interpreter.

Make _Py_FinishPendingCalls() and _PyEval_FiniThreads() functions
private.

https://bugs.python.org/issue39984

[vstinner]

I don't understand the relationship between _Py_CheckRecursionLimit and ceval.recursion_limit.

When USE_STACKCHECK macro is not defined, _Py_CheckRecursiveCall() is called if tstate->recursion_depth is greater than _Py_CheckRecursionLimit.

If USE_STACKCHECK is defined, _Py_CheckRecursiveCall() copies ceval.recursion_limit into _Py_CheckRecursionLimit:

    /* Needed for ABI backwards-compatibility (see [bpo-31857](https://bugs.python.org/issue31857)) */
    _Py_CheckRecursionLimit = recursion_limit;

See bpo-31857.

[vstinner]

See also bpo-38644: Add Py_EnterRecursiveCall() to the limited API (PR #17046).

[vstinner]

This change is more complicated than expected. I started with something simpler instead, move tracing_possible: PR #19047. I close this PR.

I may rewrite this PR on top of PR #19047 once it's merged.