Rename the no_std feature to core.

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

lib/wasm/src/lib.rs

@@ -21,7 +21,7 @@
 extern crate alloc;
 
 #[allow(unused_extern_crates)]
-#[cfg(feature = "no_std")]
+#[cfg(not(feature = "std"))]
 extern crate hashmap_core;
 
 extern crate wasmparser;