Reintroduce Variance node

[danez]

Q	A
Bug fix?	no
Breaking change?	yes
New feature?	no
Deprecations?	no
Spec compliancy?	yes
Tests added/pass?	yes
Fixed tickets	Fixes babel/babel#4722
License	MIT

This reverts the change that made variance a string literal and reintroduces the Variance node.

//cc @samwgoldman

[codecov-io]

Codecov Report

Merging #333 into 7.0 will increase coverage by 0.7%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              7.0     #333     +/-   ##
=========================================
+ Coverage   97.46%   98.17%   +0.7%     
=========================================
  Files          19       19             
  Lines        3237     4380   +1143     
  Branches      847     1398    +551     
=========================================
+ Hits         3155     4300   +1145     
+ Misses         35       34      -1     
+ Partials       47       46      -1

Impacted Files	Coverage Δ
src/plugins/flow.js	97.53% <100%> (+0.8%)	✅
src/parser/lval.js	98.55% <ø> (-0.1%)	❌
src/parser/statement.js	97.89% <ø> (-0.01%)	❌
src/util/identifier.js	100% <ø> (ø)	✅
src/index.js	100% <ø> (ø)	✅
src/plugins/jsx/index.js	98.4% <ø> (+0.62%)	✅
src/tokenizer/index.js	99.04% <ø> (+0.77%)	✅
src/parser/expression.js	98.3% <ø> (+1.85%)	✅

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