Replace hardcoded rounding modes with named for arb

[jamesnohilly]

This PR replaces instances of hardcoded rounding mode values in usages of arf_get_d with their named counterparts.

After looking through the FLINT documentation, this doesn't require any new constants as the rounding modes in ArbTypes.jl refers to the correct values from arf_get_d and uses arf_rnd_t.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.33%. Comparing base (4df8c91) to head (7275fe4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2022      +/-   ##
==========================================
- Coverage   88.36%   88.33%   -0.03%     
==========================================
  Files          99       99              
  Lines       36248    36248              
==========================================
- Hits        32029    32021       -8     
- Misses       4219     4227       +8     

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[fingolfin]

Excellent!