Use TryInto in all configurations.

Summary
-------

This commit is a step toward removing `cast.rs` (see
RCasatta#1).  This commit tweaks
`cast.rs` to uniformly use `TryInto` in all configurations (not just in
`#[cfg(debug_assertions)]`).

Benchmark impact
----------------

Disclaimers: Benchmarking is tricky:

* Confidence intervals below are relatively wide
* Code changes can have indirect/unexpected impact (e.g. noise from
  random changes to binary code layout may be significant in
  microbenchmarks).

This commit introduces additional checks into the relase configuration
and therefore may have a performance impact.  Benchmarks before this
commit:

    $ cargo bench --features=bench | grep bench
    ...
    ...bench_qr_code_with_low_ecc     ...:  18,680,806 ns/iter (+/- 147,143)
    ...bench_find_min_version         ...:           1 ns/iter (+/- 0)
    ...bench_push_splitted_bytes      ...:       3,754 ns/iter (+/- 48)
    ...bench::bench_optimize_base64   ...:      29,262 ns/iter (+/- 470)
    ...bench::bench_optimize_example1 ...:       1,582 ns/iter (+/- 33)

Benchmarks after this commit:

    $ cargo bench --features=bench | grep bench
    ...
    ...bench_qr_code_with_low_ecc     ...:  18,805,800 ns/iter (+/- 133,211)
    ...bench_find_min_version         ...:           1 ns/iter (+/- 0)
    ...bench_push_splitted_bytes      ...:       4,070 ns/iter (+/- 19)
    ...bench::bench_optimize_base64   ...:      36,202 ns/iter (+/- 1,069)
    ...bench::bench_optimize_example1 ...:       1,706 ns/iter (+/- 23)

The difference is 18,805,800 ns - 18,680,806 ns = 124,994 ns.  This
is a slowdown of 124,994 ns / 18,805,800 ns = 0.664%.

Evaluation
----------

Arguments for landing this commit:

* Potential for code removal and simplification
* Consistently applying correctness checks in all configurations.
    * This is consistent with Rust's safe-by-default approach
    * This is consistent with the recent change in Chromium policy which
      recommends against using debug-mode-only `DCHECK`s:
      https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/checks.md

Arguments against landing this commit:

* 0.664% slowdown (note the disclaimers above about micro-benchmarks)
* Extra checks may increase the binary size of the code

src/cast.rs

@@ -1,6 +1,4 @@
-#[cfg(debug_assertions)]
 use std::convert::TryInto;
-
 use std::fmt::Display;
 
 // TODO remove this, use try_into wher as_* is used
@@ -41,47 +39,23 @@ impl<T> ExpectOrOverflow for Option<T> {
 
 macro_rules! impl_as {
     ($ty:ty) => {
-        #[cfg(debug_assertions)]
         impl As for $ty {
             fn as_u16(self) -> u16 {
                 self.try_into().unwrap()
             }
-
             fn as_i16(self) -> i16 {
                 self.try_into().unwrap()
             }
-
             fn as_u32(self) -> u32 {
                 self.try_into().unwrap()
             }
-
             fn as_usize(self) -> usize {
                 self.try_into().unwrap()
             }
-
             fn as_isize(self) -> isize {
                 self.try_into().unwrap()
             }
         }
-
-        #[cfg(not(debug_assertions))]
-        impl As for $ty {
-            fn as_u16(self) -> u16 {
-                self as u16
-            }
-            fn as_i16(self) -> i16 {
-                self as i16
-            }
-            fn as_u32(self) -> u32 {
-                self as u32
-            }
-            fn as_usize(self) -> usize {
-                self as usize
-            }
-            fn as_isize(self) -> isize {
-                self as isize
-            }
-        }
     };
 }