Refactor and rename src/constraints

[odow]

Since these are implementation details (they are not exported), I'm being a bit ruthless with renaming for clarity.

One question for @ericphanson: it's not clear to me why some atoms call MOI_add_constraint, and others call a <: Constraint. Is it because we can only return duals via <: Constraint?

[codecov]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (32b487d) 92.63% compared to head (af1522c) 92.64%.

Files	Patch %	Lines
src/constraints/LessThanConstraint.jl	78.94%	8 Missing ⚠️
src/constraints/GreaterThanConstraint.jl	81.57%	7 Missing ⚠️
src/constraints/EqualToConstraint.jl	83.78%	6 Missing ⚠️
src/constraints/ExponentialConeConstraint.jl	63.63%	4 Missing ⚠️
src/problems.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
+ Coverage   92.63%   92.64%   +0.01%     
==========================================
  Files          89       91       +2     
  Lines        5171     5179       +8     
==========================================
+ Hits         4790     4798       +8     
  Misses        381      381              

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[ericphanson]

Yeah, we only have some constraints modeled as objects. Those the user can construct with operator-overloading syntax (for most of em), and they support show and duals (for the most part), and often have constructors to make it easier to construct. However the MOI model only needs an MOI constraint so if we don’t have an object defined then we sometimes just add the constraint directly instead.