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

cranelift/wasm/src/environ/spec.rs

@@ -8,8 +8,8 @@
 
 use crate::state::FuncTranslationState;
 use crate::translation_utils::{
-    DataIndex, ElemIndex, EntityType, Event, EventIndex, FuncIndex, Global, GlobalIndex, Memory,
-    MemoryIndex, Table, TableIndex, TypeIndex,
+    DataIndex, ElemIndex, EntityIndex, EntityType, Event, EventIndex, FuncIndex, Global,
+    GlobalIndex, Memory, MemoryIndex, ModuleIndex, Table, TableIndex, TypeIndex,
 };
 use core::convert::From;
 use core::convert::TryFrom;
@@ -22,6 +22,7 @@ use cranelift_frontend::FunctionBuilder;
 use serde::{Deserialize, Serialize};
 use std::boxed::Box;
 use std::string::ToString;
+use std::vec::Vec;
 use thiserror::Error;
 use wasmparser::ValidatorResources;
 use wasmparser::{BinaryReaderError, FuncValidator, FunctionBody, Operator, WasmFeatures};
@@ -969,4 +970,16 @@ pub trait ModuleEnvironment<'data>: TargetEnvironment {
     fn module_end(&mut self, index: usize) {
         drop(index);
     }
+
+    /// Indicates that this module will have `amount` instances.
+    fn reserve_instances(&mut self, amount: u32) {
+        drop(amount);
+    }
+
+    /// Declares a new instance which this module will instantiate before it's
+    /// instantiated.
+    fn declare_instance(&mut self, module: ModuleIndex, args: Vec<EntityIndex>) -> WasmResult<()> {
+        drop((module, args));
+        Err(WasmError::Unsupported("wasm instance".to_string()))
+    }
 }