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.

.github/workflows/rust.yml

@@ -59,6 +59,14 @@ jobs:
           command: test
           args: --release
 
+      - name: Run cargo build --all-features
+        uses: actions-rs/cargo@v1
+        env:
+          RUSTFLAGS: -D warnings
+        with:
+          command: build
+          args: --all-features
+
   test:
     name: Test Suite
     strategy:
@@ -88,6 +96,14 @@ jobs:
           command: test
           args: --release
 
+      - name: Run cargo build --all-features
+        uses: actions-rs/cargo@v1
+        env:
+          RUSTFLAGS: -D warnings
+        with:
+          command: build
+          args: --all-features
+
   fmt:
     name: Rustfmt
     runs-on: ubuntu-latest