Allow serializing all cranelift-module data structures (#6172)

* Remove ModuleCompiledFunction

The same information can be retrieved using

ctx.compiled_code().unwrap().code_info().total_size

In addition for Module implementations that don't immediately compile the
given function there is no correct value that can be returned.

* Don't give anonymous functions and data objects an internal name

This internal name can conflict if a module is serialized and then
deserialized into another module. It also wasn't used by any of the
Module implementations anyway.

* Allow serializing all cranelift-module data structures

This allows a Module implementation to serialize it's internal state and
deserialize it in another compilation session. For example to implement
LTO or to load the module into cranelift-interpreter.

* Use expect

cranelift/codegen/src/ir/function.rs

@@ -31,7 +31,7 @@ use super::{RelSourceLoc, SourceLoc, UserExternalName};
 
 /// A version marker used to ensure that serialized clif ir is never deserialized with a
 /// different version of Cranelift.
-#[derive(Copy, Clone, Debug, PartialEq, Hash)]
+#[derive(Default, Copy, Clone, Debug, PartialEq, Hash)]
 pub struct VersionMarker;
 
 #[cfg(feature = "enable-serde")]