This patch introduces two new functions to obtain the degree of a tree
node:

degree(node) is a normal, freestanding function that will in theory even
work for nodes where the number of children is determined at run time.

StaticDegree<Node> and staticDegree<Node> obtain the degree as a compile
time constant and are thus only usable for nodes where the degree is
fixed at compile time. staticDegree<Node> is a variable template and
thus creates a hard dependency on C++14.

Internally, both functions dispatch to the following function signature:

template<typename Node, typename NodeTag>
constexpr std::size_t degree(const Node* node, NodeTag nodeTag);

Note that this signature uses a pointer to the node; this is necessary
to make it usable in the constexpr context, where the node parameter has
to be constexpr - manufacturing a constexpr reference is not possible in
general.

The default implementation of this function calls the static member
function Node::degree(). If your node implementation requires different
behavior, you can provide an overload for your custom node tag that will
be found via ADL. staticDegree<Node> will only work if this function is
constexpr (it is thus important to make Node::degree() constexpr in your
implementation iff the degree is a compile time constant).