rust: remove support for `re2` With the performance issues with `regex` figured out and fixed in previous patches and `regex` newly gaining support for empty alternations, there is no reason to keep `re2` around anymore. It's only *marginally* faster at creating the regex which saves at most a couple of ms, but gets beaten by `regex` in every other aspect. This removes the Rust/C/C++ bridge (hooray!), the `with-re2` feature, the conditional code that goes with it, the documentation and relevant part of the debug/module output. Differential Revision: https://phab.mercurial-scm.org/D8594

--- a/rust/hg-cpython/src/debug.rs	Fri May 29 12:12:16 2020 +0200
+++ b/rust/hg-cpython/src/debug.rs	Fri May 29 12:17:59 2020 +0200
@@ -16,8 +16,6 @@
     m.add(py, "__package__", package)?;
     m.add(py, "__doc__", "Rust debugging information")?;
 
-    m.add(py, "re2_installed", cfg!(feature = "with-re2"))?;
-
     let sys = PyModule::import(py, "sys")?;
     let sys_modules: PyDict = sys.get(py, "modules")?.extract(py)?;
     sys_modules.set_item(py, dotted_name, &m)?;