Split monolithic raw crates into sys crates

Creating a crate for bwd-gc highlights the fact that it would be nice to
fix 2!

The file blocklist is a lost less unmaintainable then the more
fine-grained one we had before.

Fix #9

nix-bindings-util/src/context.rs

@@ -113,7 +113,7 @@ macro_rules! check_call_opt_key {
         {
             let ctx : &mut $crate::context::Context = $ctx;
             let ret = $($f)::*(ctx.ptr(), $($arg,)*);
-            if unsafe { raw::err_code(ctx.ptr()) == raw::err_NIX_ERR_KEY } {
+            if unsafe { $crate::raw_sys::err_code(ctx.ptr()) == $crate::raw_sys::err_NIX_ERR_KEY } {
                 ctx.clear();
                 return Ok(None);
             }