bindings: unfortunately, LTO does not seem to work...

I'm not sure why this is happening. My best guess is that it's either because:
  - the specific compiler that `cc` ends up using doesn't play nice with the bitcode that rustc/the LLVM linker plugin end up producing and expecting
     + the linker-plugin based LTO page mentions this: https://doc.rust-lang.org/rustc/linker-plugin-lto.html#toolchain-compatibility
     + also, there's actually (afaik) no easy way to externally (outside of `build.rs`) set what compiler `cc` should use (there are env vars but not ones that are specific to `cc`) and it's probably best not to assume that users will be using `clang`..
  - or, we're not actually producing the ar format necessary, but this seems unlikely

In any case, this is probably a good thing to stick behind a feature flag since the LTO setup seems somewhat brittle and we can't/shouldn't expect users to have modern versions of lld/clang.

.cargo/config

@@ -1,2 +1,3 @@
 [build]
 rustdocflags = ["--cfg", "docs"]
+rustflags = ["-Clinker-plugin-lto", "-Clinker=clang",  "-Clink-arg=-fuse-ld=lld"]

build.rs

@@ -496,7 +496,8 @@ fn main() -> Result<()> {
     }
 
     build
-        .flag_if_supported("-flto")
+        // .flag_if_supported("-flto")
+        // .flag_if_supported("-flto=thin")
         .flag_if_supported("-std=c++14")
         .flag_if_supported("-Wno-format-security")
         .cargo_metadata(true)