Add fast_type_map, use it authoritatively for local types and as a hint for global types (ABI breaking) (#5842)

* Add fast_type_map, use it authoritatively for local types and as a hint for global types

nanobind has a similar two-level lookup strategy, added and explained
by
b515b1f7f2

In this PR I've ported this approach to pybind11. To avoid an ABI
break, I've kept the fast maps to the `local_internals`. I think this
should be safe because any particular module should see its
`local_internals` reset at least as often as the global `internals`,
and misses in the fast "hint" map for global types fall back to the
global `internals`.

Performance seems to have improved. Using my patched fork of
pybind11_benchmark
(https://github.com/swolchok/pybind11_benchmark/tree/benchmark-updates,
specifically commit hash b6613d12607104d547b1c10a8145d1b3e9937266), I
run bench.py and observe the MyInt case. Each time, I do 3 runs and
just report all 3.

master, Mac: 75.9, 76.9, 75.3 nsec/loop
this PR, Mac: 73.8, 73.8, 73.6 nsec/loop
master, Linux box: 188, 187, 188 nsec/loop
this PR, Linux box: 164, 165, 164 nsec/loop

Note that the "real" percentage improvement is larger than implied by the
above because master does not yet include #5824.

* simplify unsafe_reset_local_internals in test

* pre-implement PYBIND11_INTERNALS_VERSION 12

* use PYBIND11_INTERNALS_VERSION 12 on Python 3.14 per suggestion

* Implement reviewer comments: revert PY_VERSION_HEX change, fix REVIEW comment, add two-level lookup comments. ci.yml coming separately

* Use the inplace build to smoke test ABI bump?

* [skip ci] Remove "smoke" from comment. This is full testing, just only on a few platforms.

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

.github/workflows/ci.yml

@@ -203,7 +203,8 @@ jobs:
       if: runner.os != 'Windows'
       run: echo "CMAKE_GENERATOR=Ninja" >> "$GITHUB_ENV"
 
-    # More-or-less randomly adding a few extra flags here
+    # More-or-less randomly adding a few extra flags here.
+    # In particular, use this one to test the next ABI bump (internals version).
     - name: Configure
       run: >
         cmake -S. -B.
@@ -213,6 +214,7 @@ jobs:
         -DDOWNLOAD_CATCH=ON
         -DDOWNLOAD_EIGEN=ON
         -DCMAKE_CXX_STANDARD=14
+        -DPYBIND11_INTERNALS_VERSION=10000000
 
     # Checks to makes sure defining `_` is allowed
     # Triggers EHsc missing error on Windows