This is a minor simplification of the grammar of array allocation expressions.

• ArrayAllocation nodes now have an ArrayType as a child,instead of a type + ArrayAllocationDims. This allows sharing the API and is overall less surprising (you'd expect an array allocation expression to have an array type node somewhere)
• ArrayTypeDims is renamed to ArrayDimensions (it's tricky to spot the extra s)
• ArrayDimExpr extends ArrayTypeDim. So ArrayAllocationDims can be removed, since ArrayDimensions collects all dimensions, whether they're initialized with a length expression (ArrayDimExpr) or not.

I'm not sure we really need an ArrayDimExpr node, ArrayTypeDim could as well have an optional child for the expression, but I'm concerned the API of that node would be more complex.

This prepares for fixing #997. We now have a single node type for array dimensions. We can add it in VariableDeclaratorId and MethodDeclaration, and get rid of Dimensionable. This fix has to wait for #1980 because both touch the VariableDeclaratorId production