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

tests/instantiate.rs

@@ -21,6 +21,6 @@ fn test_environ_translate() {
 
     let mut resolver = NullResolver {};
     let mut compiler = Compiler::new(isa, CompilationStrategy::Auto);
-    let instance = instantiate(&mut compiler, &data, None, &mut resolver, false);
+    let instance = instantiate(&mut compiler, &data, &mut resolver, false);
     assert!(instance.is_ok());
 }