mapreduce to map+reduce for tuples

[jishnub]

After this,

julia> s = Fourier()
Fourier(【0.0,6.283185307179586❫)

julia> @inferred ApproxFunBase.block(s, 2)
Block(1)

[codecov]

Codecov Report

Base: 71.90% // Head: 71.94% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (837d2dd) compared to base (5da41bb).
Patch coverage: 75.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #359      +/-   ##
==========================================
+ Coverage   71.90%   71.94%   +0.03%     
==========================================
  Files          79       79              
  Lines        8301     8302       +1     
==========================================
+ Hits         5969     5973       +4     
+ Misses       2332     2329       -3     

Impacted Files	Coverage Δ
src/Spaces/SumSpace.jl	71.31% <33.33%> (ø)
src/LinearAlgebra/helper.jl	64.23% <100.00%> (+0.08%)	⬆️
src/Multivariate/TensorSpace.jl	87.17% <100.00%> (+0.85%)	⬆️

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