Make it easier to use pymatching as a C++ dependency

[Strilanc]

• Add a libpymatching target to the CMakeLists.txt file
• Add support for building with bazel
  • Add a WORKSPACE file
  • Add a BUILD file
  • Try it with bazel build :all
• Add an IF(EXISTS ${CMAKE_CURRENT_LIST_DIR}/pybind11/CMakeLists.txt) around the pybind11 cmake target
  • Also included a warning message that explains how to fix the issue
  • This avoids some issues where cmake fetching a project won't recursively fetch the submodules, preventing everything from working instead of just the pybind11 target
  • Also it avoids some potential getting-started issues, or at least defers them, with getting the code to build
• Fix some constructor order warnings in DetectorNode and MatchingGraph
• Fix some implicit downcast warnings
• Bump the stim commit tag to the latest one on main, and fix the resulting breakages
  • Most fixes were adding new bitword size template parameters and switching from SparseShot.obs_mask to SparseShot.obs_mask_as_u64().

Linking from CMake

Note the commit ID I'm using is the latest commit of this PR, not one on main.

Here is now a way to use pymatching as a cmake dependency. In the CMakeLists.txt file fetch it using FetchContent:

FetchContent_Declare(pymatching
        GIT_REPOSITORY https://github.com/oscarhiggott/pymatching.git
        GIT_TAG 48098cf8d6d77fb4c2caf0ab319b31bde4d37d22 )
FetchContent_GetProperties(pymatching)
if(NOT pymatching_POPULATED)
  FetchContent_Populate(pymatching)
  add_subdirectory(${pymatching_SOURCE_DIR})
endif()

You can then link to it:

[...]
target_link_libraries(YOURLIBRARY libpymatching)
[...]

Linking from Bazel

Note the commit ID I'm using is the latest commit of this PR, not one on main.

In WORKSPACE:

load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
git_repository(
    name = "pymatching",
    commit = "48098cf8d6d77fb4c2caf0ab319b31bde4d37d22",
    remote = "https://github.com/oscarhiggott/pymatching.git",
)

In BUILD:

cc_binary(
    name = "your_project",
    ...
    deps = [
        "@pymatching//:libpymatching",
    ],
)

Fixes https://github.com/oscarhiggott/PyMatching/issues/76

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (1211298) to head (48098cf). Report is 24 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #77 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 3 3 Lines 331 331 ========================================= Hits 331 331 ```

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[Strilanc]

Don't merge this yet, I'm still cleaning it up.

[Strilanc]

Okay this should be safe to merge now, assuming tests pass.