Represent serial numbers using SerialNumber struct

In a previous commit, we changed the serial number representation from a
string to an integer.  This made it easier to compare serial numbers,
but also introduced new problems:
- Serial numbers should be formatted consistently, for example as
  "{:#010x}".  It is hard to ensure this for an integer value.
- The format of the serial number may be subject to change.  Users
  should not rely too much on the u32 representation.

Therefore we introduce a new SerialNumber struct that represents a
serial number.  Currently it only stores a u32 value.  The following
traits and functions can be used to access its value:
- FromStr for string parsing
- ToString/Display for string formatting
- as_u32 to access the underlying integer value

1 files changed, 4 insertions, 3 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c94e170..9e675f0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,11 +5,12 @@ SPDX-License-Identifier: CC0-1.0
 
 # Unreleased
 - Add `String` value to the `Error::UnexpectedError` variant.
-- Always store serial numbers as integers:
+- Always store serial numbers as structs:
+  - Introduce the `SerialNumber` struct.
   - Change the return type of `Device::get_serial_number` from `Result<String,
-    _>` to `Result<u32, _>`.
+    _>` to `Result<SerialNumber, _>`.
   - Change the type of the field `DeviceInfo.serial_number` from
-    `Option<String>` to `Option<u32>`.
+    `Option<String>` to `Option<SerialNumber>`.
 - Use `NK_get_status` instead of `NK_read_config` to implement the
   `Device::get_config` function.