Improve SourceCodeLocation accuracy.

[iflan]

CssDeclarationNode did not have the SourceCodeLocation as a constructor parameter, nor did it preserve it on a deep copy.

CssNodesListNode now tries to build its locations from its constituent nodes unless a SourceCodeLocation is set explicitly.

Several bugs in the actual parser are also fixed as are numerous problems in FixupFontDeclarations.

There are new tests and a SourceCodeLocationSubject for Truth which should make new tests easier to write.