Export MOI shortcut

[blegat]

This PR exports MOI from JuMP so that users of JuMP do not have to copy paste

using MathOptInterface
const MOI = MathOptInterface

everywhere

This was suggested by @odow

[mlubin]

This doesn't seem right. Having JuMP export a mysterious object named "MOI" doesn't help with readability of the code. Scipy doesn't export np as numpy. We should consider re-exporting the important enums from MOI like optimization sense and statuses.

[codecov]

Codecov Report

Merging #1549 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1549   +/-   ##
=======================================
  Coverage   89.48%   89.48%           
=======================================
  Files          27       27           
  Lines        3623     3623           
=======================================
  Hits         3242     3242           
  Misses        381      381

Impacted Files	Coverage Δ
src/JuMP.jl	76.19% <ø> (ø)	⬆️

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