Code Tweaks

Add MessagePropertiesBuilder convenience class for creating message properties.
Add additional tests.
Update sys version.

More complete implementation

• Now has interfaces for direct methods and cloud to device messages.
• Exposes functions to get and set the prerequisite failure function (default just loops 0 process::abort works much better)
• Stop the sys layer types leaking into the user's code. This was mostly return codes which are not mirrored by a Rust emum so that the sys crate is not required by a app using this crate.

Add low level functions

To allow better control over memory added low level functions that require the caller to provide the needed buffers. The high level will allocate those with an arbitrary and then shrink them. They will always work since they will loop until the buffer is large enough. In the low level versions all that is the responsibility of the caller.

Added tests.

First drop

First drop of stable code. A long way from complete but enough to use for a simple example.

Done:

• Get MQTT client id
• Get MQTT user id
• Get MQTT telemetry publish topic
• Get SAS key
• Get MQTT password from generated SAS token
• Build tested on Windows 10 and Ubuntu 20.04

Not Done:

• No support for cloud to device messages
• No support for direct messages
• No support for device twins
• No support for X.509 authentication
• No support for DPS