Improve handling of strings for backtraces (#843)

* Improve handling of strings for backtraces

Largely avoid storing strings at all in the `wasmtime-*` internal
crates, and instead only store strings in a separate global cache
specific to the `wasmtime` crate itself. This global cache is inserted
and removed from dynamically as modules are created and deallocated, and
the global cache is consulted whenever a `Trap` is created to
symbolicate any wasm frames.

This also avoids the need to thread `module_name` through the jit crates
and back, and additionally removes the need for `ModuleSyncString`.

* Run rustfmt

crates/api/src/trampoline/create_handle.rs

@@ -4,7 +4,7 @@ use crate::runtime::Store;
 use anyhow::Result;
 use std::any::Any;
 use std::collections::HashSet;
-use std::rc::Rc;
+use std::sync::Arc;
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::wasm::DefinedFuncIndex;
 use wasmtime_environ::Module;
@@ -37,7 +37,7 @@ pub(crate) fn create_handle(
         .unwrap_or_else(PrimaryMap::new);
 
     Ok(InstanceHandle::new(
-        Rc::new(module),
+        Arc::new(module),
         finished_functions.into_boxed_slice(),
         imports,
         &data_initializers,