Refactor module instantiation in the runtime.

This commit refactors module instantiation in the runtime to allow for
different instance allocation strategy implementations.

It adds an `InstanceAllocator` trait with the current implementation put behind
the `OnDemandInstanceAllocator` struct.

The Wasmtime API has been updated to allow a `Config` to have an instance
allocation strategy set which will determine how instances get allocated.

This change is in preparation for an alternative *pooling* instance allocator
that can reserve all needed host process address space in advance.

This commit also makes changes to the `wasmtime_environ` crate to represent
compiled modules in a way that reduces copying at instantiation time.

crates/environ/src/data_structures.rs

@@ -20,7 +20,7 @@ pub mod isa {
 }
 
 pub mod entity {
-    pub use cranelift_entity::{packed_option, BoxedSlice, EntityRef, PrimaryMap};
+    pub use cranelift_entity::{packed_option, BoxedSlice, EntityRef, EntitySet, PrimaryMap};
 }
 
 pub mod wasm {