assembler_parsert: construct with message handler

[tautschnig]

This both avoids an object of static lifetime as well as it fixes the (transitive) use of the deprecated messaget() constructor.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (69bb2b6) 79.09% compared to head (9495ab2) 79.09%.

Files	Patch %	Lines
src/assembler/assembler_parser.cpp	75.00%	2 Missing ⚠️
src/assembler/assembler_parser.h	66.66%	1 Missing ⚠️
src/cbmc/cbmc_parse_options.cpp	50.00%	1 Missing ⚠️
src/goto-analyzer/goto_analyzer_parse_options.cpp	50.00%	1 Missing ⚠️
src/goto-diff/goto_diff_parse_options.cpp	50.00%	1 Missing ⚠️
.../goto-instrument/goto_instrument_parse_options.cpp	66.66%	1 Missing ⚠️
src/goto-synthesizer/cegis_verifier.cpp	50.00%	1 Missing ⚠️
src/libcprover-cpp/api.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8139      +/-   ##
===========================================
- Coverage    79.09%   79.09%   -0.01%     
===========================================
  Files         1695     1695              
  Lines       196583   196586       +3     
===========================================
+ Hits        155496   155497       +1     
- Misses       41087    41089       +2     

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[feliperodri]

LGTM.

[martin-cs]

Thanks for the clean up.