Enhance type stability with extra type parameter

[wheeheee]

For FFTAPlan_re and FFTAPlan_cx, plus a bit of cleanup. Slightly edited test_allocations, which is still a bit wonky because apparently when testing checkbounds=true causes allocations, althought that's technically a Julia problem. Maybe better to remove those tests altogether.
For compat, an extra constructor that takes only the first two params.
sort! also feels somewhat questionable when e.g. 2:-1:1 can be passed as the region argument

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.22%. Comparing base (f4c4fb8) to head (f6e99e1).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/plan.jl	93.75%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
- Coverage   97.21%   96.22%   -0.99%     
==========================================
  Files           4        4              
  Lines         431      424       -7     
==========================================
- Hits          419      408      -11     
- Misses         12       16       +4     

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[dannys4]

For my own reference, what's the effect of this change? I know basic macros, and I'm guessing that the point is to ensure the @allocations symbol doesn't appear in code run for Julia <1.9, but I was under the impression that the revised code amounts to the same thing.

[wheeheee]

@static causes the if statement to be evaluated only once during macro expansion, so after that the macro would be the code in only one of the branches. I suspected weird "action-at-a-distance" machinery was involved in allocating so I wanted to minimise extraneous code paths.

That was also the motivation for escaping the @allocations macro.
How much this actually works, not clear, but it's not harmful or incorrect and the tests pass with this change.

[dannys4]

lgtm