Improve error handling, and start refactoring Instance.

Introduce proper error handling in several places, and perform a first
pass at refactoring Instance to make it easier to use.

lib/execute/src/imports.rs

@@ -0,0 +1,30 @@
+use cranelift_entity::PrimaryMap;
+use cranelift_wasm::{FuncIndex, GlobalIndex, MemoryIndex, TableIndex};
+use vmcontext::{VMGlobal, VMMemory, VMTable};
+
+/// Resolved import pointers.
+#[derive(Debug)]
+pub struct Imports {
+    /// Resolved addresses for imported functions.
+    pub functions: PrimaryMap<FuncIndex, *const u8>,
+
+    /// Resolved addresses for imported tables.
+    pub tables: PrimaryMap<TableIndex, *mut VMTable>,
+
+    /// Resolved addresses for imported globals.
+    pub globals: PrimaryMap<GlobalIndex, *mut VMGlobal>,
+
+    /// Resolved addresses for imported memories.
+    pub memories: PrimaryMap<MemoryIndex, *mut VMMemory>,
+}
+
+impl Imports {
+    pub fn new() -> Self {
+        Self {
+            functions: PrimaryMap::new(),
+            tables: PrimaryMap::new(),
+            globals: PrimaryMap::new(),
+            memories: PrimaryMap::new(),
+        }
+    }
+}