Display triangular matrices with dots

[jishnub]

After this, the structured zeros of a UpperTriangularToeplitz are displayed with dots, as in a UpperTriangular:

julia> U = UpperTriangularToeplitz([1,2,3,4])
4×4 UpperTriangularToeplitz{Int64, Vector{Int64}}:
 1  2  3  4
 ⋅  1  2  3
 ⋅  ⋅  1  2
 ⋅  ⋅  ⋅  1

This involves extending Base.replace_in_print_matrix, which is internal to Base (although it should perhaps be public).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (02f06e5) 95.61% compared to head (82469f9) 95.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   95.61%   95.62%   +0.01%     
==========================================
  Files           9        9              
  Lines         957      961       +4     
==========================================
+ Hits          915      919       +4     
  Misses         42       42              

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[github-actions]

Pull Request Test Coverage Report for Build 7470652756

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.63%

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Totals
Change from base Build 7470644076:	0.02%
Covered Lines:	919
Relevant Lines:	961

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💛 - Coveralls