Add a futures-v0_3 feature flag that gates access to the Stream implementation

Author: bdash

Cargo.lock

@@ -19,11 +19,15 @@ edition = "2018"
 travis-ci = { repository = "vorner/signal-hook" }
 maintenance = { status = "actively-developed" }
 
+[features]
+futures-v0_3 = ["futures-core-0_3"]
+
 [dependencies]
 libc = "~0.2"
 signal-hook = { version = "~0.3", path = ".." }
-futures = "~0.3"
+futures-core-0_3 = { package = "futures-core", version = "~0.3", optional = true }
 tokio = { version = "~1", features = ["net"] }
+cfg-if = "~1"
 
 [dev-dependencies]
 tokio = { package = "tokio", version = "~1", features = ["full"] }