Change pixels vector to tuple in fits_read_pix

[jishnub]

This was changed in v1.7.1 as using a Vector instead of a Tuple improves type-stability, but since there is a function barrier, the instability does not appear to matter for performance. Using a Tuple leads to zero allocations, even though this introduces a type-instability.

julia> @btime fits_read_pix($f, $A); # vector
  147.441 ns (2 allocations: 80 bytes)

julia> @btime fits_read_pix($f, $A); # tuple
  139.501 ns (0 allocations: 0 bytes)

Opening this PR to discuss what people prefer.

Alternately, we may define a custom Ones vector implementation analogous to FillArrays, which would be type-stable as well as allocation-free.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.27%. Comparing base (b8cc1f0) to head (33a4eb9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #83   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files           1        1           
  Lines         889      889           
=======================================
  Hits          847      847           
  Misses         42       42           

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