Rename the no_std feature to core.

See
https://github.com/yurydelendik/wasmparser.rs/pull/49#issuecomment-375436225
for more details.

lib/cretonne/Cargo.toml

@@ -26,9 +26,9 @@ version = "0.1.1"
 optional = true
 
 [features]
-# The "std" feature enables use of libstd. The "no_std" feature enables use
+# The "std" feature enables use of libstd. The "core" 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"]
+core = ["hashmap_core"]