Instantiate nested modules for module linking (#2447)

This commit implements the interpretation necessary of the instance
section of the module linking proposal. Instantiating a module which
itself has nested instantiated instances will now instantiate the nested
instances properly. This isn't all that useful without the ability to
alias exports off the result, but we can at least observe the side
effects of instantiation through the `start` function.

cc #2094

tests/all/wast.rs

@@ -13,6 +13,7 @@ fn run_wast(wast: &str, strategy: Strategy) -> anyhow::Result<()> {
     let simd = wast.iter().any(|s| s == "simd");
 
     let multi_memory = wast.iter().any(|s| s == "multi-memory");
+    let module_linking = wast.iter().any(|s| s == "module-linking");
     let bulk_mem = multi_memory || wast.iter().any(|s| s == "bulk-memory-operations");
 
     // Some simd tests assume support for multiple tables, which are introduced
@@ -24,6 +25,7 @@ fn run_wast(wast: &str, strategy: Strategy) -> anyhow::Result<()> {
         .wasm_bulk_memory(bulk_mem)
         .wasm_reference_types(reftypes)
         .wasm_multi_memory(multi_memory)
+        .wasm_module_linking(module_linking)
         .strategy(strategy)?
         .cranelift_debug_verifier(true);