Make is_exact_type and is_domain_type more convenient

[fingolfin]

For examples, this now works:

julia> is_domain_type(ZZ)
true

julia> is_domain_type(ZZ(2))
true

julia> is_domain_type(residue_ring(ZZ,6)[1])
false

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.19%. Comparing base (9843ed5) to head (28b0cb3).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Rings.jl	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1942      +/-   ##
==========================================
- Coverage   88.21%   88.19%   -0.02%     
==========================================
  Files         119      119              
  Lines       30385    30391       +6     
==========================================
  Hits        26803    26803              
- Misses       3582     3588       +6     

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