Add an explicit std feature so that features are purely additive.

lib/cretonne/Cargo.toml

@@ -17,14 +17,19 @@ name = "cretonne"
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
 # accomodated in `tests`.
+
 [dependencies.hashmap_core]
 version = "0.1.1"
 optional = true
+
 [dependencies.error_core]
 version = "0.1.0"
 optional = true
 
 [features]
-# Currently, the only feature is the `no_std` feature.
-# Enabling this disables use of `stdlib`.
-no_std = ["hashmap_core", "error_core"]
+# The "std" feature enables use of libstd. The "no_std" feature enables use
+# of some minimal std-like replacement libraries. At least one of these two
+# features to be enabled.
+default = ["std"]
+std = []
+no_std = ["hashmap_core", "error_core"]