Reduce command duplication

[puddly]

This PR finishes the "derives from the previous version" format that we've been using for EZSP v8-v14.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.15%. Comparing base (91b8f37) to head (18d1d52).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #635   +/-   ##
=======================================
  Coverage   99.15%   99.15%           
=======================================
  Files          50       50           
  Lines        3412     3447   +35     
=======================================
+ Hits         3383     3418   +35     
  Misses         29       29           

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[puddly]

I believe that 18d1d52 is going to be a safe change: I've not seen a single SDK version where the sent command parameter is actually 16 bits. The SDK uses 16-bit integer constants but the command is always serialized and deserialized to/from an 8-bit integer. I think the fact that EZSP is lenient with command payload lengths and that we use little endian encoding has sort of "hidden" this problem:

In [1]: (0x0012).to_bytes(2, 'little')
Out[1]: b'\x12\x00'

In [2]: (0x0012).to_bytes(1, 'little')
Out[2]: b'\x12'