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.

cranelift/wasm/src/module_translator.rs

@@ -101,6 +101,8 @@ pub fn translate_module<'data>(
 
             Payload::DataCountSection { count, range } => {
                 validator.data_count_section(count, &range)?;
+
+                // NOTE: the count here is the total segment count, not the passive segment count
                 environ.reserve_passive_data(count)?;
             }