Ensure C++ compatibility for designated initialisers

[tlsa]

Resolves #166.

C++ finally has designated initialisers but they still require
member order to match the definition, even though the point of
designated initilisers is to allow you to not care about order
and be explicit about specific members' values.

Reorder the members in the schema building macros to allow
them to be used from a modern enough C++ version.

[codecov]

Codecov Report

Merging #167 (187117d) into main (3d234d8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   97.11%   97.11%           
=======================================
  Files           9        9           
  Lines        1596     1596           
  Branches      342      342           
=======================================
  Hits         1550     1550           
  Misses         24       24           
  Partials       22       22           

Flag	Coverage Δ
unittests	97.11% <ø> (ø)

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