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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.