speed up MTK OOP vector usage

[ChrisRackauckas]

No description provided.

[codecov]

Codecov Report

Merging #213 into master will decrease coverage by 0.11%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   89.16%   89.04%   -0.12%     
==========================================
  Files          15       15              
  Lines         489      493       +4     
==========================================
+ Hits          436      439       +3     
- Misses         53       54       +1

Impacted Files	Coverage Δ
src/utils.jl	89.85% <87.5%> (-0.92%)	⬇️

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