MultiServer: speed up location commands (#1926)

* MultiServer: speed up location commands

Adds optimized pure python wrapper around locations dict
Adds optimized cython implementation of the wrapper, saving cpu time and 80% memory use

* Speedups: auto-build on import and build during setup

* Speedups: add requirements

* CI: don't break with build_ext

* Speedups: use C++ compiler for pyximport

* Speedups: cleanup and more validation

* Speedups: add tests for LocationStore

* Setup: delete temp in-place build modules

* Speedups: more tests and safer indices

The change has no security implications, but ensures that entries[IndexEntry.start] is always valid.

* Speedups: add cython3 compatibility

* Speedups: remove unused import

* Speedups: reformat

* Speedup: fix empty set in test

* Speedups: use regular dict in Locations.get_for_player

* CI: run unittests with beta cython

now with 2x nicer names

_speedups.pyxbld

@@ -0,0 +1,8 @@
+# This file is required to get pyximport to work with C++.
+# Switching from std::set to a pure C implementation is still on the table to simplify everything.
+
+def make_ext(modname, pyxfilename):
+    from distutils.extension import Extension
+    return Extension(name=modname,
+                     sources=[pyxfilename],
+                     language='c++')