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/sections_translator.rs

@@ -10,9 +10,9 @@
 use crate::environ::{ModuleEnvironment, WasmError, WasmResult};
 use crate::state::ModuleTranslationState;
 use crate::translation_utils::{
-    tabletype_to_type, type_to_type, DataIndex, ElemIndex, EntityType, Event, EventIndex,
-    FuncIndex, Global, GlobalIndex, GlobalInit, Memory, MemoryIndex, Table, TableElementType,
-    TableIndex, TypeIndex,
+    tabletype_to_type, type_to_type, DataIndex, ElemIndex, EntityIndex, EntityType, Event,
+    EventIndex, FuncIndex, Global, GlobalIndex, GlobalInit, InstanceIndex, Memory, MemoryIndex,
+    ModuleIndex, Table, TableElementType, TableIndex, TypeIndex,
 };
 use crate::wasm_unsupported;
 use core::convert::TryFrom;
@@ -475,3 +475,37 @@ pub fn parse_name_section<'data>(
     }
     Ok(())
 }
+
+/// Parses the Instance section of the wasm module.
+pub fn parse_instance_section<'data>(
+    section: wasmparser::InstanceSectionReader<'data>,
+    environ: &mut dyn ModuleEnvironment<'data>,
+) -> WasmResult<()> {
+    environ.reserve_types(section.get_count())?;
+
+    for instance in section {
+        let instance = instance?;
+        let module = ModuleIndex::from_u32(instance.module());
+        let args = instance
+            .args()?
+            .into_iter()
+            .map(|result| {
+                let (kind, idx) = result?;
+                Ok(match kind {
+                    ExternalKind::Function => EntityIndex::Function(FuncIndex::from_u32(idx)),
+                    ExternalKind::Table => EntityIndex::Table(TableIndex::from_u32(idx)),
+                    ExternalKind::Memory => EntityIndex::Memory(MemoryIndex::from_u32(idx)),
+                    ExternalKind::Global => EntityIndex::Global(GlobalIndex::from_u32(idx)),
+                    ExternalKind::Module => EntityIndex::Module(ModuleIndex::from_u32(idx)),
+                    ExternalKind::Instance => EntityIndex::Instance(InstanceIndex::from_u32(idx)),
+                    ExternalKind::Event => unimplemented!(),
+
+                    // this won't pass validation
+                    ExternalKind::Type => unreachable!(),
+                })
+            })
+            .collect::<WasmResult<Vec<_>>>()?;
+        environ.declare_instance(module, args)?;
+    }
+    Ok(())
+}