Add a CommandPtr with an internal unique_ptr to enable not needing to move the underlying classes, which is error-prone due to the potential for lambda captures.
Assorted improvements to the ergonomics of declaring requirements in the new
command framework. C++ requirements list parameters have been defaulted
to an empty list, some missing C++ requirements list parameters have been
added, and both C++ and Java have been given requirements list params in
various InstantCommand wrapper methods (#2049), whose value is
forwarded to the command.
