Use mutable copies in transform/itransform

[jishnub]

After this, the following works:

julia> values(Fun(Chebyshev(), 1.0:2.0))
2-element Vector{Float64}:
  2.414213562373095
 -0.41421356237309515

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c676431) 70.71% compared to head (2c13ac8) 65.10%.
Report is 6 commits behind head on master.

Files	Patch %	Lines
src/Space.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
- Coverage   70.71%   65.10%   -5.62%     
==========================================
  Files          80       80              
  Lines        8398     8432      +34     
==========================================
- Hits         5939     5490     -449     
- Misses       2459     2942     +483     

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