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/module_environ.rs

@@ -1,5 +1,5 @@
 use crate::func_environ::FuncEnvironment;
-use crate::module::{Export, MemoryPlan, Module, ModuleSyncString, TableElements, TablePlan};
+use crate::module::{Export, MemoryPlan, Module, TableElements, TablePlan};
 use crate::tunables::Tunables;
 use cranelift_codegen::ir;
 use cranelift_codegen::ir::{AbiParam, ArgumentPurpose};
@@ -371,7 +371,10 @@ impl<'data> cranelift_wasm::ModuleEnvironment<'data> for ModuleEnvironment<'data
     }
 
     fn declare_func_name(&mut self, func_index: FuncIndex, name: &'data str) -> WasmResult<()> {
-        self.result.module.func_names[func_index] = ModuleSyncString::new(Some(name));
+        self.result
+            .module
+            .func_names
+            .insert(func_index, name.to_string());
         Ok(())
     }
 }