SL-18330: In XML formatter, avoid adding call stack depth.

[nat-goodspeed]

Making LLSDXMLFormatter._elt() accept a lambda was a tricky way to minimize
source changes to existing _elt() calls in _ARRAY() and _MAP(). The trouble is
that that added function entries for each level of a deeply-nested LLSD
structure -- and with our users, we unfortunately do encounter deeply-nested
large inventories. Log observed RecursionError failures in AIS.

Explicitly write out the individual sequence of calls in _ARRAY(), _MAP() and
the top-level _write(), the only callers to pass lambdas to _elt().

[github-actions]

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ @nat-goodspeed
❌ @LogLinden
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[codecov]

Codecov Report

Merging #13 (674f8aa) into main (ed393fd) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   90.28%   90.33%   +0.04%     
==========================================
  Files           6        6              
  Lines         844      848       +4     
==========================================
+ Hits          762      766       +4     
  Misses         82       82              

Impacted Files	Coverage Δ
llsd/serde_xml.py	96.94% <100.00%> (+0.09%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[bennettgoble]

Changes seem fine. Since Log is an employee I think we can look past the CLA check for now.

[nat-goodspeed]

@bennettgoble I'm not yet clear on what should trigger a new release. I guess I'll create a minor one and hope it helps.

[LogLinden]

I have read the CLA Document and I hereby sign the CLA