Pre-release v0.9.0-2019.03.01.00

Changes since v0.9.0-2019.02.11.00:

New functionality:

• Add S2ShapeIndex Dump function
• Add Init to Graph nested classes
• S2Polygon::InitToOperation: Propagate S2Error

Bug fixes:

• Fix S2BooleanOperation empty/full handling
• Minor compilation fixes for nacl
• Fix LaxPolygonLayer bug
• Fix rare bug in S2BooleanOperation
• Fix rare bug in VisitCrossingEdgePairs
• Fix UB discovered by -D_GLIBCXX_DEBUG
• S2Cap::Add: Fix bug with empty other cap
• OwningShape: Fix potential memory leak
• S2Builder: Clear layer_is_full_polygon_predicates_
• S2EdgeTesselator: Improve error estimation