Option for host managed memory (#1400)

* Option for host managed memory

* Rename Allocator to MemoryCreator

* Create LinearMemory and MemoryCreator traits in api

* Leave only one as_ptr function in LinearMemory trait

* Memory creator test

* Update comments/docs for LinearMemory and MemoryCreator traits

* Add guard page to the custom memory example

* Remove mut from LinearMemory trait as_ptr

* Host_memory_grow test

crates/api/src/instance.rs

@@ -31,6 +31,7 @@ fn instantiate(
                 config.validating_config.operator_config.enable_bulk_memory,
                 &mut resolver,
                 sig_registry,
+                config.memory_creator.as_ref().map(|a| a as _),
             )
             .map_err(|e| -> Error {
                 match e {