Do not discard const qualifiers from pointers

[jamesjer]

GCC issues several warnings of this form:

/builddir/build/BUILD/flint-3.4.0-build/flint-3.4.0/src/arf/io.c: In function ‘arf_load_str’:
/builddir/build/BUILD/flint-3.4.0-build/flint-3.4.0/src/arf/io.c:96:11: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   96 |     e_str = strchr(data, ' ');
      |           ^

This PR fixes all of them by adding const qualifiers as needed.

[albinahlback]

Looks good, thanks!