Avoid MArrays with FiniteDiff backend

[dingraha]

I noticed some allocations when using FiniteDiff with StaticArrays that I eventually traced to calls to similar in the FiniteDiff extension. Adding the check for immutable input or output arrays seemed to fix this. Haven't attempted to add tests for this—wanted to get some feedback before diving in. What do you think?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.60%. Comparing base (cbfeb7c) to head (5c8e123).

❗ There is a different number of reports uploaded between BASE (cbfeb7c) and HEAD (5c8e123). Click for more details.

HEAD has 118 uploads less than BASE

Flag	BASE (cbfeb7c)	HEAD (5c8e123)
DIT	25	4
DI	116	19

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1019       +/-   ##
===========================================
- Coverage   98.14%   87.60%   -10.54%     
===========================================
  Files         138      135        -3     
  Lines        8131     8078       -53     
===========================================
- Hits         7980     7077      -903     
- Misses        151     1001      +850     

Flag	Coverage Δ
DI	86.77% <100.00%> (-12.10%)	⬇️
DIT	89.83% <ø> (-6.40%)	⬇️

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[gdalle]

Nice catch! The alternative would be replacing similar with copy, does that work on SArrays too?