cleanup most warnings in the build

the two remaining ones look like ... perhaps actual structural
flaws in Ring, so it's something to contemplate how to fix.

src/c2rust/ecp_nistz.rs

@@ -19,10 +19,10 @@ pub type crypto_word_t = uint32_t;
 pub type Limb = crypto_word_t;
 #[no_mangle]
 pub unsafe extern "C" fn little_endian_bytes_from_scalar(
-    mut str: *mut uint8_t,
-    mut str_len: size_t,
-    mut scalar: *const Limb,
-    mut num_limbs: size_t,
+    str: *mut uint8_t,
+    str_len: size_t,
+    scalar: *const Limb,
+    num_limbs: size_t,
 ) {
     if str_len
         == num_limbs
@@ -40,7 +40,7 @@ pub unsafe extern "C" fn little_endian_bytes_from_scalar(
             .as_ptr(),
         );
     }
-    let mut i: size_t = 0;
+    let mut i: size_t;
     i = 0 as core::ffi::c_int as size_t;
     while i < num_limbs.wrapping_mul(core::mem::size_of::<Limb>() as u32) {
         let mut d: Limb =

src/c2rust/gfp_p256.rs

@@ -34,9 +34,9 @@ static mut N_N0: [BN_ULONG; 2] = [
 ];
 #[no_mangle]
 pub unsafe extern "C" fn p256_scalar_mul_mont(
-    mut r: *mut Limb,
-    mut a: *const Limb,
-    mut b: *const Limb,
+    r: *mut Limb,
+    a: *const Limb,
+    b: *const Limb,
 ) {
     bn_mul_mont(
         r,
@@ -49,9 +49,9 @@ pub unsafe extern "C" fn p256_scalar_mul_mont(
 }
 #[no_mangle]
 pub unsafe extern "C" fn p256_scalar_sqr_rep_mont(
-    mut r: *mut Limb,
-    mut a: *const Limb,
-    mut rep: Limb,
+    r: *mut Limb,
+    a: *const Limb,
+    rep: Limb,
 ) {
     p256_scalar_mul_mont(r, a, a);
     let mut i: Limb = 1 as core::ffi::c_int as Limb;