Fix unused imports in some configurations

Author: rafalh

src/io.rs

@@ -1,6 +1,4 @@
 use crate::error::IoError;
-#[cfg(all(not(feature = "std"), feature = "alloc"))]
-use alloc::vec::Vec;
 
 /// Provides IO error as an associated type.
 ///

src/time.rs

@@ -1,3 +1,4 @@
+#[cfg(feature = "chrono")]
 use core::convert::TryFrom;
 use core::fmt::Debug;