Refactor parse_number_range to avoid spurious uninitialized warning

[tautschnig]

g++-7 -O3 spuriously warned that begin_range was accessed without being
initialized:

../src/util/format_number_range.cpp:102:5: error: ‘*((void*)& begin_range +4)’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     for(unsigned i = *begin_range; i < *number; ++i)
     ^~~
../src/util/format_number_range.cpp:121:23: note: ‘*((void*)& begin_range +4)’ was declared here
   optionalt<unsigned> begin_range;
                       ^~~~~~~~~~~
cc1plus: all warnings being treated as errors

Although optionalt may seem a bit more beautiful, it's perfectly
possible to implement parse_number_range without any use of optionalt.

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• 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

Merging #6017 (ca6f109) into develop (36151cf) will decrease coverage by 0.09%.
The diff coverage is 97.50%.

@@             Coverage Diff             @@
##           develop    #6017      +/-   ##
===========================================
- Coverage    74.23%   74.13%   -0.10%     
===========================================
  Files         1444     1444              
  Lines       157309   157322      +13     
===========================================
- Hits        116781   116637     -144     
- Misses       40528    40685     +157     

Impacted Files	Coverage Δ
jbmc/src/java_bytecode/java_bytecode_typecheck.cpp	75.00% <ø> (ø)
..._bytecode_convert_class/convert_abstract_class.cpp	100.00% <ø> (ø)
...va_bytecode_convert_method/convert_initalizers.cpp	100.00% <ø> (ø)
...ambdas/java_bytecode_parse_lambda_method_table.cpp	100.00% <ø> (ø)
...va_bytecode/java_replace_nondet/replace_nondet.cpp	83.33% <ø> (ø)
src/ansi-c/gcc_types.cpp	88.37% <ø> (ø)
src/goto-instrument/cover.cpp	83.42% <ø> (ø)
src/goto-instrument/goto_program2code.cpp	69.14% <ø> (ø)
src/goto-programs/json_goto_trace.cpp	81.74% <ø> (ø)
src/linking/static_lifetime_init.cpp	95.89% <ø> (ø)
... and 12 more

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