Preserve types in triangular constructors

[jishnub]

Currently,

julia> U = UpperTriangularToeplitz([1,2,3,4])
4×4 UpperTriangularToeplitz{Int64, Vector{Int64}}:
 1  2  3  4
 0  1  2  3
 0  0  1  2
 0  0  0  1

julia> UpperTriangular(U)
4×4 UpperTriangularToeplitz{Int64, Vector{Int64}}:
 1  2  3  4
 0  1  2  3
 0  0  1  2
 0  0  0  1

This is not ideal, as the UpperTriangular constructor should create an object of that type. This PR changes this to

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

This wrapper would allow one to use optimized LinearAlgebra functions that are defined for UpperTriangular/LowerTriangular, although there is a certain degree of redundancy present.

Similarly, change Symmetric(::AbstractToeplitz) to return a wrapper instead of a SymmetricToeplitz.

Since this was explicitly implemented this way and was being tested for, I have marked this change as breaking (minor version update). This also allows us to bump the Julia compatibility and drop support for v1.0. This should close #91.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.05%. Comparing base (ee9e796) to head (5cd64ec).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   93.87%   94.05%   +0.17%     
==========================================
  Files           9        9              
  Lines         963      958       -5     
==========================================
- Hits          904      901       -3     
+ Misses         59       57       -2     

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[github-actions]

Pull Request Test Coverage Report for Build 7443282799

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 95.781%

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Totals
Change from base Build 7443258038:	0.2%
Covered Lines:	908
Relevant Lines:	948

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