Resolve collapsible_match clippy lint

    warning: this `if let` can be collapsed into the outer `if let`
        --> gen/src/write.rs:1117:13
         |
    1117 | /             if let Type::RustBox(_) | Type::UniquePtr(_) | Type::Str(_) | Type::SliceRef(_) = ret {
    1118 | |                 write!(out, ")");
    1119 | |             }
         | |_____________^
         |
    help: the outer pattern can be modified to include the inner pattern
        --> gen/src/write.rs:1116:21
         |
    1116 |         if let Some(ret) = &sig.ret {
         |                     ^^^ replace this binding
    1117 |             if let Type::RustBox(_) | Type::UniquePtr(_) | Type::Str(_) | Type::SliceRef(_) = ret {
         |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ with this pattern
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match
         = note: `#[warn(clippy::collapsible_match)]` on by default

gen/src/write.rs

@@ -1113,10 +1113,10 @@ fn write_rust_function_shim_impl(
     }
     write!(out, ")");
     if !indirect_return {
-        if let Some(ret) = &sig.ret {
-            if let Type::RustBox(_) | Type::UniquePtr(_) | Type::Str(_) | Type::SliceRef(_) = ret {
-                write!(out, ")");
-            }
+        if let Some(Type::RustBox(_) | Type::UniquePtr(_) | Type::Str(_) | Type::SliceRef(_)) =
+            &sig.ret
+        {
+            write!(out, ")");
         }
     }
     writeln!(out, ";");