[outcome] Inline GDB pretty printing support

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2024-07-01 14:45:36

Dear Boost,

The biggest new feature to land into Outcome since it went ABI stable is 
inline GDB pretty printing support, whereby we embed into all binaries 
using Outcome the Python script to display the state of 
`result<>/outcome<>`. As Outcome uses a layered inheritance technique to 
match triviality of member functions from its configured types, having 
to click down through the layers to discover the state was always 
frustrating. MSVC has long shipped with a visualiser to eliminate this, 
so now support for GDB matches MSVC support.

The relevant implementation file can be found at:

https://github.com/boostorg/outcome/blob/master/include/boost/outcome/outcome_gdb.h

I would appreciate any testing and reporting of any issues or bugs 
before the 1.86 bugfix window closes. It should "just work" once you 
have enabled inline script loading in your `.gdbinit`.

Thanks,
Niall