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.

Cargo.toml

@@ -7,7 +7,6 @@ application providing general-purpose public-key signing and encryption
 with hardware-backed private keys for RSA (2048/1024) and ECC (P-256/P-384)
 algorithms (e.g, PKCS#1v1.5, ECDSA)
 """
-
 authors    = ["Tony Arcieri <bascule@gmail.com>", "Yubico AB"]
 edition    = "2018"
 license    = "BSD-2-Clause"
@@ -16,16 +15,25 @@ readme     = "README.md"
 categories = ["api-bindings", "cryptography", "hardware-support"]
 keywords   = ["ccid", "ecdsa", "rsa", "piv", "yubikey"]
 
+[badges]
+maintenance = { status = "experimental" }
+
 [dependencies]
 des = "0.3"
 getrandom = "0.1"
-hmac = "0.7"
+hmac = { version = "0.7", optional = true }
 log = "0.4"
-pbkdf2 = "0.3"
+pbkdf2 = { version = "0.3", optional = true }
 pcsc = "2"
-sha-1 = "0.8"
-subtle = "2"
+sha-1 = { version = "0.8", optional = true }
+subtle = { version = "2", optional = true }
 zeroize = "1"
 
 [dev-dependencies]
 env_logger = "0.7"
+
+[features]
+untested = ["hmac", "pbkdf2", "sha-1", "subtle"]
+
+[package.metadata.docs.rs]
+all-features = true