Current modes are `LOG` and `LISTING`, which `log()` and store for conversion to json respectively.
Add `ContentListing` listing struct to (recursively) contain help data for conversion to a json object to be exported and used elsewhere (e.g. the docs).
Rather than formatting as rst we can just export with type information and do the conversion at the destination (i.e. in the python code which loads the domain for autodoc).
Implement `PrettyHelp::has_content()`.
Provide `PrettyHelp::get_content()` which returns a read-only list of the current content.
`PrettyHelp` constructor takes optional `Mode` enum to define format of help content.
Updates `PrettyHelp` methods to use a switch case for checking current mode, calling `log_abort()` in the default case (i.e. unsupported mode).
Use `std::experimental::source_location` because clang support is `??`
Add `ENABLE_SOURCE_LOCATION` make variable and corresponding `YOSYS_ENABLE_SOURCE_LOCATION` define.
Dummy out the struct if disabled and check for null instead of using `#ifdef` blocks everywhere.
Define `PrettyHelp` class with methods for declaring different parts of help message.
Currently able to produce standard help messages as expected.
Updates chformal to use (only) the new help_v2.
Currently makes use of a global static to track the current help context, allowing register.h to live in blissful ignorance and instead rely on help_v2 implementations calling `auto *help = PrettyHelp::get_current();` and `return true;` to minimise impact on rebuilds (i.e. not requiring every source file to be recompiled).
Krystine Sherwin