Make WasiCtxBuilder by-ref.

This commit makes `WasiCtxBuilder` take `&mut Self` and return `&mut
Self` for its methods.  This is needed to allow for the same
(unmoved) `WasiCtxBuilder` to be used when building a WASI context.

Also fixes up the C API to remove the unnecessary `Box::from_raw` and
`forget` calls which were previously needed for the moving version of
`WasiCtxBuilder`.

src/commands/run.rs

@@ -303,22 +303,25 @@ impl ModuleRegistry {
         argv: &[String],
         vars: &[(String, String)],
     ) -> Result<ModuleRegistry> {
-        let mut cx1 = wasi_common::WasiCtxBuilder::new()
-            .inherit_stdio()
-            .args(argv)
-            .envs(vars);
+        let mut cx1 = wasi_common::WasiCtxBuilder::new();
+
+        cx1.inherit_stdio().args(argv).envs(vars);
+
         for (name, file) in preopen_dirs {
-            cx1 = cx1.preopened_dir(file.try_clone()?, name);
+            cx1.preopened_dir(file.try_clone()?, name);
         }
+
         let cx1 = cx1.build()?;
 
         let mut cx2 = wasi_common::old::snapshot_0::WasiCtxBuilder::new()
             .inherit_stdio()
             .args(argv)
             .envs(vars);
+
         for (name, file) in preopen_dirs {
             cx2 = cx2.preopened_dir(file.try_clone()?, name);
         }
+
         let cx2 = cx2.build()?;
 
         Ok(ModuleRegistry {