Change how Instance stores instantiated memories in the runtime.

This commit changes `Instance` such that memories can be stored statically,
with just a base pointer, size, maximum, and a callback to make memory
accessible.

Previously the memories were being stored as boxed trait objects, which would
require the pooling allocator to do some unpleasant things to avoid
allocations.

With this change, the pooling allocator can simply define a memory for the
instance without using a trait object.

crates/runtime/src/instance.rs

@@ -4,7 +4,7 @@
 
 use crate::export::Export;
 use crate::externref::{StackMapRegistry, VMExternRefActivationsTable};
-use crate::memory::{RuntimeLinearMemory, RuntimeMemoryCreator};
+use crate::memory::{Memory, RuntimeMemoryCreator};
 use crate::table::{Table, TableElement};
 use crate::traphandlers::Trap;
 use crate::vmcontext::{
@@ -51,7 +51,7 @@ pub(crate) struct Instance {
     offsets: VMOffsets,
 
     /// WebAssembly linear memory data.
-    memories: PrimaryMap<DefinedMemoryIndex, Box<dyn RuntimeLinearMemory>>,
+    memories: PrimaryMap<DefinedMemoryIndex, Memory>,
 
     /// WebAssembly table data.
     tables: PrimaryMap<DefinedTableIndex, Table>,