cryptography: add std feature

Adds an off-by-default `std` feature which activates the `std` features
of all of the other dependencies.

This is a somewhat lousy way of doing this, and I wish that there were a
better approach, so much that I posted a Pre-Pre-RFC about it:

https://internals.rust-lang.org/t/pre-pre-rfc-weak-cargo-feature-activation/13141

For now though, this is immediately useful to me.

cryptography/Cargo.toml

@@ -21,5 +21,17 @@ signature = { version = "1.2.0", optional = true, default-features = false, path
 stream-cipher = { version = "0.7", optional = true, path = "../stream-cipher" }
 universal-hash = { version = "0.4", optional = true, path = "../universal-hash" }
 
+[features]
+std = [
+    "aead/std",
+    "block-cipher/std",
+    "digest/std",
+    "elliptic-curve/std",
+    "mac/std",
+    "signature/std",
+    "stream-cipher/std",
+    "universal-hash/std"
+]
+
 [package.metadata.docs.rs]
 all-features = true