Reduce number of unsafe as conversions

Enable clippy::cast_lossless lint and get rid of
some conversions pointed out by  clippy::as_conversions.

src/lib.rs

@@ -7,7 +7,8 @@
     clippy::all,
     clippy::indexing_slicing,
     clippy::wildcard_imports,
-    clippy::needless_borrow
+    clippy::needless_borrow,
+    clippy::cast_lossless
 )]
 #![allow(
     clippy::match_bool,