Change how flags are stored in serialized modules.

This commit changes how both the shared flags and ISA flags are stored in the
serialized module to detect incompatibilities when a serialized module is
instantiated.

It improves the error reporting when a compiled module has mismatched shared
flags.

crates/environ/src/data_structures.rs

@@ -11,7 +11,7 @@ pub mod ir {
 
 pub mod settings {
     pub use cranelift_codegen::settings::{
-        builder, Builder, Configurable, Flags, OptLevel, SetError, Setting, SettingKind,
+        builder, Builder, Configurable, Flags, OptLevel, SetError, Setting, SettingKind, Value,
     };
 }