Be explicit about example feature requirements

The examples depend on having the "std" feature [1]. In preparation for
being able to run tests with `--no-default-features` add the "std"
feature as a requirement for all three examples. While we are at it use
the correct rand feature requirement: `rand-std`.

[1] Technically we only need "alloc" but "alloc" is not working with
Rust 1.29 currently so just use "std".

Cargo.toml

@@ -49,14 +49,15 @@ rand = { version = "0.6", features = ["wasm-bindgen"] }
 
 [[example]]
 name = "sign_verify_recovery"
-required-features = ["recovery"]
+required-features = ["std", "recovery"]
 
 [[example]]
 name = "sign_verify"
+required-features = ["std"]
 
 [[example]]
 name = "generate_keys"
-required-features = ["rand"]
+required-features = ["std", "rand-std"]
 
 [workspace]
 members = ["secp256k1-sys"]

contrib/test.sh

@@ -45,9 +45,9 @@ if [ "$DO_FEATURE_MATRIX" = true ]; then
     fi
 
     # Examples
-    cargo run --example sign_verify
-    cargo run --example sign_verify_recovery --features=recovery
-    cargo run --example generate_keys --features=rand
+    cargo run --example sign_verify --features=std
+    cargo run --example sign_verify_recovery --features=std,recovery
+    cargo run --example generate_keys --features=std,rand-std
 fi
 
 # Docs