refactor: rename DifferentialWasmiModuleConfig to SingleFunctionModuleConfig

Since we plan to reuse this configuration, we rename it and ensure it
has at least 1 type (this resulted in invalid modules).

fuzz/fuzz_targets/differential_wasmi.rs

@@ -5,7 +5,7 @@ use wasmtime_fuzzing::{generators, oracles};
 
 fuzz_target!(|data: (
     generators::Config,
-    wasm_smith::ConfiguredModule<oracles::DifferentialWasmiModuleConfig>
+    wasm_smith::ConfiguredModule<oracles::SingleFunctionModuleConfig>
 )| {
     let (config, mut wasm) = data;
     wasm.ensure_termination(1000);