rename user-facing functions for style compliance

[mlubin]

Mostly focused on user-facing functions to break the important parts first. Includes optimize -> optimize! for consistency with MOI.

Closes #1382

[codecov]

Codecov Report

Merging #1442 into master will decrease coverage by 0.01%.
The diff coverage is 98.56%.

@@            Coverage Diff             @@
##           master    #1442      +/-   ##
==========================================
- Coverage   88.87%   88.86%   -0.02%     
==========================================
  Files          25       25              
  Lines        3695     3691       -4     
==========================================
- Hits         3284     3280       -4     
  Misses        411      411

Impacted Files	Coverage Δ
src/optimizerinterface.jl	73.91% <ø> (ø)	⬆️
src/nlp.jl	82.01% <0%> (ø)	⬆️
src/operators.jl	88.3% <100%> (ø)	⬆️
src/macros.jl	89.64% <100%> (ø)	⬆️
src/quadexpr.jl	90.69% <100%> (ø)	⬆️
src/print.jl	80.54% <100%> (-0.42%)	⬇️
src/parseexpr.jl	94.09% <100%> (ø)	⬆️
src/affexpr.jl	95.27% <100%> (ø)	⬆️
src/variables.jl	88.37% <100%> (ø)	⬆️
src/JuMP.jl	77.03% <87.5%> (ø)	⬆️

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[blegat]

I agree with optimize -> optimize!. The fact that it does not remove the solution but modifies its internals to contain the solution also apply here