don't use mul! inplace

[jishnub]

This fixes a somewhat serious bug, albeit one that might not be frequently encountered (no wonder it lurked undetected)
On master:

julia> v = rand(4);

julia> R = Conversion(Chebyshev(), Legendre());

julia> ApproxFunBase.mul_coefficients!(R, copy(v))
4-element Vector{Float64}:
 0.0
 0.0
 0.0
 0.0

After this PR

julia> ApproxFunBase.mul_coefficients!(R, copy(v))
4-element Vector{Float64}:
 0.41517303921077103
 0.17527749035532603
 0.7202004312320129
 1.35520616568446

julia> ApproxFunBase.mul_coefficients!(R, copy(v)) == ApproxFunBase.mul_coefficients(R, v)
true

[codecov]

Codecov Report

Patch coverage: 74.89% and project coverage change: -3.89 ⚠️

Comparison is base (2f401fa) 71.22% compared to head (0c6638a) 67.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   71.22%   67.34%   -3.89%     
==========================================
  Files          80       81       +1     
  Lines        8331     8468     +137     
==========================================
- Hits         5934     5703     -231     
- Misses       2397     2765     +368     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	84.00% <ø> (+0.66%)	⬆️
src/LinearAlgebra/blas.jl	77.77% <ø> (ø)
src/Spaces/Spaces.jl	45.23% <ø> (-21.83%)	⬇️
src/Operators/qr.jl	57.01% <22.22%> (-3.70%)	⬇️
src/Spaces/SumSpace.jl	65.99% <33.33%> (-3.69%)	⬇️
src/Operators/SubOperator.jl	81.28% <60.00%> (-1.89%)	⬇️
src/eigen.jl	66.66% <66.66%> (ø)
src/Operators/functionals/CalculusFunctional.jl	61.53% <73.33%> (-10.88%)	⬇️
src/Operators/banded/CalculusOperator.jl	72.13% <77.41%> (-2.44%)	⬇️
src/show.jl	85.34% <78.04%> (+2.50%)	⬆️
... and 6 more

... and 44 files with indirect coverage changes

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