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/environ/src/lib.rs

@@ -54,7 +54,7 @@ pub use crate::func_environ::BuiltinFunctionIndex;
 #[cfg(feature = "lightbeam")]
 pub use crate::lightbeam::Lightbeam;
 pub use crate::module::{
-    Export, MemoryPlan, MemoryStyle, Module, ModuleSyncString, TableElements, TablePlan, TableStyle,
+    Export, MemoryPlan, MemoryStyle, Module, TableElements, TablePlan, TableStyle,
 };
 pub use crate::module_environ::{
     translate_signature, DataInitializer, DataInitializerLocation, FunctionBodyData,