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/jit/src/context.rs

@@ -109,13 +109,7 @@ impl Context {
         self.validate(&data).map_err(SetupError::Validate)?;
         let debug_info = self.debug_info();
 
-        instantiate(
-            &mut *self.compiler,
-            &data,
-            None,
-            &mut self.namespace,
-            debug_info,
-        )
+        instantiate(&mut *self.compiler, &data, &mut self.namespace, debug_info)
     }
 
     /// Return the instance associated with the given name.
@@ -146,7 +140,7 @@ impl Context {
         self.validate(&data).map_err(SetupError::Validate)?;
         let debug_info = self.debug_info();
 
-        CompiledModule::new(&mut *self.compiler, data, None, debug_info)
+        CompiledModule::new(&mut *self.compiler, data, debug_info)
     }
 
     /// If `name` isn't None, register it for the given instance.