ctutils: impl From<u8> for Choice (#1309)

This is the main way `subtle` constructs `Choice` so it's important for
backwards compatibility.

It seems like if we had `from_u8_lsb`, we could avoid the panic
condition by always masking the input `u8`. We don't yet, but this
commit also leaves a comment to consider deprecating both this
`From<u8>` impl and `Choice::new` in the future.

Author: tarcieri

ctutils/src/choice.rs

@@ -49,6 +49,7 @@ impl Choice {
     ///
     /// # Panics
     /// - in `debug` builds, panics if the value is anything other than `0` or `1`.
+    // TODO(tarcieri): deprecate this in favor of non-panicking constructors?
     #[inline]
     pub const fn new(value: u8) -> Self {
         // Compare to what should be the non-secret upper bits of the value, which should always be
@@ -388,6 +389,17 @@ impl CtSelect for Choice {
     }
 }
 
+/// Create a new [`Choice`] from the given `u8` value, which MUST be either `0` or `1`.
+///
+/// # Panics
+/// - in `debug` builds, panics if the value is anything other than `0` or `1`.
+// TODO(tarcieri): deprecate this in favor of non-panicking constructors?
+impl From<u8> for Choice {
+    fn from(value: u8) -> Self {
+        Choice::new(value)
+    }
+}
+
 impl From<Choice> for u8 {
     fn from(choice: Choice) -> u8 {
         choice.to_u8()