Preserve reference order in generated XML encoders

[rallep71]

While testing SASL2, I found that generated encoders can emit groups of child
elements in a different order from the one declared in the codec
specification.

group_refs/1 currently stores the groups in a dict and returns them with
dict:to_list/1. That does not preserve their order in #elem.refs. The
encoder then builds an accumulator which is reversed before output, so the
resulting sibling order depends on the dictionary ordering.

This change records the first occurrence of each group while keeping the
existing grouping behaviour. It also folds the groups in the direction needed
by the final accumulator reversal.

The XML-RPC codec has been regenerated, and the regression test uses a small
codec with two mechanism children followed by inline. Before the fix it is
serialised as inline, mechanism, mechanism; with the fix the declared order
is retained.

Related to processone/ejabberd#4606.

Tests

• Confirmed that the new test fails against 1.1.60 and passes with this change
• rebar3 eunit -v — 86 tests passed on Erlang/OTP 28
• rebar3 xref
• REBAR=rebar3 make spec

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[p1bot]

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