tril/triu for ZerosMatrix and OneElementMatrix

[jishnub]

With this, tril/triu are no-ops for a ZerosMatrix, and O(1) for a OneElementMatrix

julia> @btime triu($(Ref(Zeros(100,100)))[]);
  2.019 μs (0 allocations: 0 bytes) # master
  2.945 ns (0 allocations: 0 bytes) # PR

julia> @btime triu($(Ref(OneElement(3, (100,100), (100,100))))[]);
  25.277 μs (2 allocations: 78.17 KiB) # master
  3.780 ns (0 allocations: 0 bytes) # PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (545fa12) to head (93edb5d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #334   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           8        8           
  Lines        1055     1063    +8     
=======================================
+ Hits         1054     1062    +8     
  Misses          1        1           

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[jishnub]

Gentle bump