Rebased version of reverted #109

[greimel]

My initial PR #109 was merged and then reverted

out of concern with memory allocation

by @eloualiche. I benchmarked this PR vs master (using the middle regression model in benchmarks/benchmarks.jl

# PR
julia> @btime reg($df, @formula(y ~ x1 + x2 + fe(id1) + fe(id2)))
  2.842 s (38190 allocations: 97.22 MiB)

# master
@btime reg($df, @formula(y ~ x1 + x2 + fe(id1) + fe(id2)))
  2.713 s (35578 allocations: 90.64 MiB)

Is this difference significant enough to prevent this feature?

[greimel]

I've gotten rid of the excess allocations.

[codecov-commenter]

Codecov Report

Merging #205 (672dd78) into master (2662649) will decrease coverage by 0.63%.
The diff coverage is 89.36%.

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
- Coverage   96.08%   95.45%   -0.64%     
==========================================
  Files           8        8              
  Lines         588      638      +50     
==========================================
+ Hits          565      609      +44     
- Misses         23       29       +6     

Impacted Files	Coverage Δ
src/FixedEffectModels.jl	100.00% <ø> (ø)
src/FixedEffectModel.jl	92.99% <76.19%> (-0.56%)	⬇️
src/partial_out.jl	91.66% <84.00%> (-3.86%)	⬇️
src/fit.jl	96.31% <97.91%> (+0.06%)	⬆️

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[nilshg]

Can this be closed as missing is now supported?