Support transforms with quasi matrices

[dlfivefifty]

This enables e.g.

T = Chebyshev()
x = axes(T,1)
T \ [exp.(x) cos.(x)]
T \ cos.(x .* (0:1000)')

[codecov]

Codecov Report

Merging #122 (ccb338d) into master (e366725) will increase coverage by 0.12%.
The diff coverage is 98.93%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   90.90%   91.03%   +0.12%     
==========================================
  Files           7        7              
  Lines         550      591      +41     
==========================================
+ Hits          500      538      +38     
- Misses         50       53       +3     

Impacted Files	Coverage Δ
src/bases/bases.jl	91.16% <98.75%> (-2.44%)	⬇️
src/ContinuumArrays.jl	87.50% <100.00%> (+4.16%)	⬆️
src/bases/splines.jl	96.34% <100.00%> (+6.21%)	⬆️
src/maps.jl	94.52% <100.00%> (+0.07%)	⬆️
src/operators.jl	76.36% <100.00%> (+1.85%)	⬆️
src/plotting.jl	82.35% <100.00%> (ø)

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