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/runtime/src/vmcontext.rs

@@ -750,7 +750,7 @@ impl VMContext {
     }
 }
 
-///
+/// Trampoline function pointer type.
 pub type VMTrampoline = unsafe extern "C" fn(
     *mut VMContext,        // callee vmctx
     *mut VMContext,        // caller vmctx