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/environ/src/module_environ.rs

@@ -1,4 +1,4 @@
-use crate::module::{MemoryPlan, Module, ModuleType, TableElements, TablePlan};
+use crate::module::{Instance, MemoryPlan, Module, ModuleType, TableElements, TablePlan};
 use crate::tunables::Tunables;
 use cranelift_codegen::ir;
 use cranelift_codegen::ir::{AbiParam, ArgumentPurpose};
@@ -6,8 +6,8 @@ use cranelift_codegen::isa::TargetFrontendConfig;
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::{
     self, translate_module, DataIndex, DefinedFuncIndex, ElemIndex, EntityIndex, EntityType,
-    FuncIndex, Global, GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table, TableIndex,
-    TargetEnvironment, TypeIndex, WasmError, WasmFuncType, WasmResult,
+    FuncIndex, Global, GlobalIndex, Memory, MemoryIndex, ModuleIndex, SignatureIndex, Table,
+    TableIndex, TargetEnvironment, TypeIndex, WasmError, WasmFuncType, WasmResult,
 };
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
@@ -60,7 +60,7 @@ pub struct ModuleTranslation<'data> {
 
     /// Indexes into the returned list of translations that are submodules of
     /// this module.
-    pub submodules: Vec<usize>,
+    pub submodules: PrimaryMap<ModuleIndex, usize>,
 
     code_index: u32,
 }
@@ -649,6 +649,18 @@ and for re-adding support for interface types you can see this issue:
         self.result.submodules.push(self.results.len());
         self.results.push(finished);
     }
+
+    fn reserve_instances(&mut self, amt: u32) {
+        self.result.module.instances.reserve(amt as usize);
+    }
+
+    fn declare_instance(&mut self, module: ModuleIndex, args: Vec<EntityIndex>) -> WasmResult<()> {
+        self.result
+            .module
+            .instances
+            .push(Instance::Instantiate { module, args });
+        Ok(())
+    }
 }
 
 /// Add environment-specific function parameters.