Rename GenericConstraint into Constraint and remove abstract type

[blegat]

This is the last breaking change I'd like to make until v0.16

[odow]

So this has the potential to be quite breaking. Is anyone writing new constraints in the wild?

[ericphanson]

I think it is unlikely; I just checked the 12 registered packages that use Convex.jl (via https://juliahub.com/ui/Packages/General/Convex), and none of them seem to. Also, I don't think it was ever documented how to construct a new Constraint type.

[odow]

Okay, then I'm 👍 for this

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.04%. Comparing base (8ad02eb) to head (d268e90).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
+ Coverage   97.94%   98.04%   +0.09%     
==========================================
  Files          89       89              
  Lines        5165     5160       -5     
==========================================
  Hits         5059     5059              
+ Misses        106      101       -5     

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