Reworked the error messages for more heplfulness.

Renamed the cast_possible_overflow lint to cast_possible_truncation,
and updated the error message, readme and crate root accordingly.
Added some more information to the message for the cast_precision_loss
lint.
Updated the test case to reflect changes.

src/lib.rs

@@ -106,7 +106,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         strings::STRING_ADD,
         strings::STRING_ADD_ASSIGN,
         types::BOX_VEC,
-        types::CAST_POSSIBLE_OVERFLOW,
+        types::CAST_POSSIBLE_TRUNCATION,
         types::CAST_PRECISION_LOSS,
         types::CAST_SIGN_LOSS,
         types::LET_UNIT_VALUE,