refactor: Add tox_options_set_savedata, deprecate old functions.

[iphydf]

This one is simpler than the existing tox_options_set_savedata_data and
therefore safer for clients to use. It will either fail (no change) or
succeed (allocation+copy).

Also setting data size without setting data makes no sense, so that
function is now deprecated.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 65.38462% with 9 lines in your changes missing coverage. Please review.

Project coverage is 72.02%. Comparing base (7c9fa37) to head (dabae03).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
toxcore/tox_options.c	69.56%	7 Missing ⚠️
testing/fuzzing/toxsave_fuzz_test.cc	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2840      +/-   ##
==========================================
- Coverage   72.14%   72.02%   -0.12%     
==========================================
  Files         152      152              
  Lines       31182    31192      +10     
==========================================
- Hits        22496    22467      -29     
- Misses       8686     8725      +39     

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