Add lld to Nix for faster builds; Remove redundant import

compiler/builtins/bitcode/src/lib.rs

@@ -3,8 +3,6 @@
 #![crate_type = "lib"]
 #![no_std]
 
-use libm;
-
 /// TODO replace this with a normal Inkwell build_cast call - this was just
 /// used as a proof of concept for getting bitcode importing working!
 #[no_mangle]

shell.nix

@@ -35,6 +35,8 @@ let
       pkgs.libffi
       pkgs.libxml2
       pkgs.zlib
+      # faster builds - see /~https://github.com/rtfeldman/roc/blob/trunk/BUILDING_FROM_SOURCE.md#use-lld-for-the-linker
+      pkgs.lld_9
     ];
 in pkgs.mkShell {
   buildInputs = inputs ++ darwin-frameworks;