Fix unneeded mutability

The `#![cfg_attr(test, deny(warnings))]` caused CI failures from
`error: variable does not need to be mutable`, which would otherwise
just be a warning.  Every case here is simply binding a `&mut`, so the
variables themselves indeed don't need to be mutable.

src/backtrack.rs

@@ -100,7 +100,7 @@ impl<'a, 'm, 'r, 's, I: Input> Bounded<'a, 'm, 'r, 's, I> {
         start: usize,
     ) -> bool {
         let mut cache = cache.borrow_mut();
-        let mut cache = &mut cache.backtrack;
+        let cache = &mut cache.backtrack;
         let start = input.at(start);
         let mut b = Bounded {
             prog: prog,

src/dfa.rs

@@ -464,7 +464,7 @@ impl<'a> Fsm<'a> {
         at: usize,
     ) -> Result<usize> {
         let mut cache = cache.borrow_mut();
-        let mut cache = &mut cache.dfa;
+        let cache = &mut cache.dfa;
         let mut dfa = Fsm {
             prog: prog,
             start: 0, // filled in below
@@ -497,7 +497,7 @@ impl<'a> Fsm<'a> {
         at: usize,
     ) -> Result<usize> {
         let mut cache = cache.borrow_mut();
-        let mut cache = &mut cache.dfa_reverse;
+        let cache = &mut cache.dfa_reverse;
         let mut dfa = Fsm {
             prog: prog,
             start: 0, // filled in below
@@ -531,7 +531,7 @@ impl<'a> Fsm<'a> {
     ) -> Result<usize> {
         debug_assert!(matches.len() == prog.matches.len());
         let mut cache = cache.borrow_mut();
-        let mut cache = &mut cache.dfa;
+        let cache = &mut cache.dfa;
         let mut dfa = Fsm {
             prog: prog,
             start: 0, // filled in below

src/pikevm.rs

@@ -109,7 +109,7 @@ impl<'r, I: Input> Fsm<'r, I> {
         start: usize,
     ) -> bool {
         let mut cache = cache.borrow_mut();
-        let mut cache = &mut cache.pikevm;
+        let cache = &mut cache.pikevm;
         cache.clist.resize(prog.len(), prog.captures.len());
         cache.nlist.resize(prog.len(), prog.captures.len());
         let at = input.at(start);
@@ -341,7 +341,7 @@ impl<'r, I: Input> Fsm<'r, I> {
                     ip = inst.goto1;
                 }
                 Match(_) | Char(_) | Ranges(_) | Bytes(_) => {
-                    let mut t = &mut nlist.caps(ip);
+                    let t = &mut nlist.caps(ip);
                     for (slot, val) in t.iter_mut().zip(thread_caps.iter()) {
                         *slot = *val;
                     }