Organize imports (#188)

* Organize imports

Use rustfmt to organize imports.  Resolve #162

* remove unused imports

* cargo fmt

src/backends/plonky2/mock/mainpod/statement.rs

@@ -1,6 +1,7 @@
+use std::fmt;
+
 use anyhow::{anyhow, Result};
 use serde::{Deserialize, Serialize};
-use std::fmt;
 
 use crate::middleware::{
     self, AnchoredKey, NativePredicate, Params, Predicate, StatementArg, ToFields,