Rename copy! -> copy_to

[blegat]

Also rename defaultcopy! to default_copy_to.

Related to #475

[mlubin]

Style question: should we define copy_to or extend Base.copyto!?

[odow]

Define copy_to. Internal consistency over using multiple dispatch just for the sake of it.

[codecov-io]

Codecov Report

Merging #491 into master will not change coverage.
The diff coverage is 84.61%.

@@           Coverage Diff           @@
##           master     #491   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files          46       46           
  Lines        4681     4681           
=======================================
  Hits         4465     4465           
  Misses        216      216

Impacted Files	Coverage Δ
src/attributes.jl	96.66% <ø> (ø)	⬆️
src/constraints.jl	100% <ø> (ø)	⬆️
src/MathOptInterface.jl	0% <ø> (ø)	⬆️
src/Utilities/copy.jl	93.75% <0%> (ø)	⬆️
src/Bridges/bridgeoptimizer.jl	96.93% <100%> (ø)	⬆️
src/Utilities/mockoptimizer.jl	94.9% <100%> (ø)	⬆️
src/Utilities/universalfallback.jl	99.3% <100%> (ø)	⬆️
src/Test/modellike.jl	100% <100%> (ø)	⬆️
src/Utilities/model.jl	97.84% <100%> (ø)	⬆️
src/Utilities/cachingoptimizer.jl	90.05% <100%> (ø)	⬆️

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[mlubin]

I'm not fully sure about not extending the Base method, but let's just go ahead and see how it turns out.

[blegat]

The copy here is not really identical since we need to return a variable map. If some copies are done silently because we overwrite Base.copyto! it might indicate an unexpected copy and hence unexpected bugs and/or inefficiency.