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

crates/jit/src/instantiate.rs

@@ -17,7 +17,7 @@ use thiserror::Error;
 use wasmtime_debug::create_gdbjit_image;
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::isa::TargetIsa;
-use wasmtime_environ::wasm::{DefinedFuncIndex, SignatureIndex};
+use wasmtime_environ::wasm::{DefinedFuncIndex, ModuleIndex, SignatureIndex};
 use wasmtime_environ::{
     CompileError, DataInitializer, DataInitializerLocation, FunctionAddressMap, Module,
     ModuleEnvironment, ModuleTranslation, StackMapInformation, TrapInformation,
@@ -71,6 +71,10 @@ pub struct CompilationArtifacts {
 
     /// Debug info presence flags.
     debug_info: bool,
+
+    /// Where to find this module's submodule code in the top-level list of
+    /// modules.
+    submodules: PrimaryMap<ModuleIndex, usize>,
 }
 
 impl CompilationArtifacts {
@@ -98,6 +102,7 @@ impl CompilationArtifacts {
                 let ModuleTranslation {
                     module,
                     data_initializers,
+                    submodules,
                     ..
                 } = translation;
 
@@ -118,6 +123,7 @@ impl CompilationArtifacts {
                     obj: obj.into_boxed_slice(),
                     unwind_info: unwind_info.into_boxed_slice(),
                     data_initializers,
+                    submodules,
                     funcs: funcs
                         .into_iter()
                         .map(|(_, func)| FunctionInfo {
@@ -336,6 +342,12 @@ impl CompiledModule {
     pub fn code(&self) -> &Arc<ModuleCode> {
         &self.code
     }
+
+    /// Returns where the specified submodule lives in this module's
+    /// array-of-modules (store at the top-level)
+    pub fn submodule_idx(&self, idx: ModuleIndex) -> usize {
+        self.artifacts.submodules[idx]
+    }
 }
 
 /// Similar to `DataInitializer`, but owns its own copy of the data rather