memory optimize the ImplicitEulerExtrapolation cache

[ChrisRackauckas]

No description provided.

[ChrisRackauckas]

@saurabhkgp21 I realized it's much easier to just explain it by showing it. Go ahead and do the same thing with the second order extrapolations.

[coveralls]

Coverage increased (+0.01%) to 78.897% when pulling 6c5f074 on memory into 280bddd on master.

[codecov]

Codecov Report

Merging #875 into master will increase coverage by 0.33%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #875      +/-   ##
==========================================
+ Coverage   80.12%   80.46%   +0.33%     
==========================================
  Files          92       92              
  Lines       30328    30699     +371     
==========================================
+ Hits        24300    24701     +401     
+ Misses       6028     5998      -30

Impacted Files	Coverage Δ
src/perform_step/extrapolation_perform_step.jl	77.15% <100%> (ø)	⬆️
src/caches/extrapolation_caches.jl	88.19% <75%> (-1.53%)	⬇️
src/tableaus/rosenbrock_tableaus.jl	98.87% <0%> (-0.31%)	⬇️
src/generic_rosenbrock.jl	98.08% <0%> (-0.22%)	⬇️
src/caches/adams_bashforth_moulton_caches.jl	80.7% <0%> (+0.44%)	⬆️
src/caches/firk_caches.jl	97.77% <0%> (+1.22%)	⬆️
src/caches/kencarp_kvaerno_caches.jl	97.02% <0%> (+1.98%)	⬆️
src/integrators/integrator_utils.jl	87.84% <0%> (+2.67%)	⬆️
src/caches/sdirk_caches.jl	96.52% <0%> (+3.47%)	⬆️
src/caches/bdf_caches.jl	89.54% <0%> (+3.92%)	⬆️
... and 2 more

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