The `ptml` namespace contains utilities for easier PTML manipulation.
These include:
* A few constants (e.g. standard attributes) available in
`revng/PTML/Constants.h`
* The `Tag` class to easily construct html/xml tags with attributes
without having to use format strings.
* The `PTMLIndentedOstream`, a llvm::raw_ostream wrapper that
automatically adds tagged indentation to the output