implement Base.copy

[mzgubic]

Closes #101

julia> @time b + D;
  0.000035 seconds (103 allocations: 29.953 KiB)

The fallback method in Base resorts to looping over eachindex, including the off-diagonal ones.

[codecov]

Codecov Report

Merging #104 (2723bb0) into master (b2ee90a) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   96.11%   95.95%   -0.17%     
==========================================
  Files           5        5              
  Lines         335      321      -14     
==========================================
- Hits          322      308      -14     
  Misses         13       13              

Impacted Files	Coverage Δ
src/blockdiagonal.jl	86.15% <100.00%> (-0.21%)	⬇️
src/linalg.jl	95.34% <0.00%> (-0.21%)	⬇️
src/base_maths.jl	100.00% <0.00%> (ø)

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