Add untested Cargo feature for untested functionality

This adds an `untested` feature to any functions which have not yet been
tested live against a YubiKey device (which is presently pretty much
everything).

This sets a clear expectation of what is presently supported, and
additionally documents the status in the README (and a series of GitHub
issues).

Adds a `cargo build --all-features` to GitHub Actions' `test` step in
order to make sure that `untested` functionality still compiles.

src/apdu.rs

@@ -71,6 +71,7 @@ impl APDU {
     }
 
     /// Set this APDU's class
+    #[cfg(feature = "untested")]
     pub fn cla(&mut self, value: u8) -> &mut Self {
         self.cla = value;
         self
@@ -83,6 +84,7 @@ impl APDU {
     }
 
     /// Set both parameters for this APDU
+    #[cfg(feature = "untested")]
     pub fn params(&mut self, p1: u8, p2: u8) -> &mut Self {
         self.p1 = p1;
         self.p2 = p2;