Hardware tree iterator tools

Summary

Add iteration tools to conveniently loop over nodes in the hardware tree.

Iterator

There will be one iterator, recursive_iterator, accessed as a boost::range from the parent class. Its intended usage is as follows (types expanded for demonstration purposes only, real-world example would use auto):

for (const std::shared_ptr<node> &child: oh.recursive_children()) {

This will be symmetric with the parent class accessor for children:

for (const std::shared_ptr<node> &child: oh.children()) {

Filter

There will be a range filter (using boost::range since std::range is C++ 20):

for (const vfat &vfat: oh.children() | filtered_children<vfat>) {

Note that:

• The filter dereferences the pointer
• According to the current naming convention, filtered_children should be spelled as FILTERED_CHILDREN because it's a static variable (templated, and of an empty class). However the above syntax works much better in lowercase.