Store v128 as u128 in wasmtime crate (#689)

As [suggested], this seems like a better and more ergonomic idea than
using `[u8; 16]`!

[suggested]: 3d69e04659 (r36326017)

crates/interface-types/src/lib.rs

@@ -157,7 +157,7 @@ impl ModuleData {
                     wasmtime::Val::I64(i) => RuntimeValue::I64(i),
                     wasmtime::Val::F32(i) => RuntimeValue::F32(i),
                     wasmtime::Val::F64(i) => RuntimeValue::F64(i),
-                    wasmtime::Val::V128(i) => RuntimeValue::V128(i),
+                    wasmtime::Val::V128(i) => RuntimeValue::V128(i.to_le_bytes()),
                     _ => panic!("unsupported value {:?}", v),
                 })
                 .collect::<Vec<RuntimeValue>>(),