The "command" encoder was mostly intended for use in `%rename` - most
uses can be achieved using the "regex" encoder, so we recommend using
that instead.
The "command" encoder suffers from a number of issues - as the
documentation for it admitted, "[it] is extremely slow compared to all
the other [encoders] as it involves spawning a separate process and
using it for many declarations is not recommended" and that it "should
generally be avoided because of performance considerations".
But it's also not portable. The design assumes that `/bin/sh` supports
`<<<` but that's a bash-specific feature so it doesn't work on platforms
where `/bin/sh` is not bash - it fails on Debian, Ubuntu and probably
some other Linux distros, plus most non-Linux platforms. Microsoft
Windows doesn't even have a /bin/sh as standard.
Finally, no escaping of the passed string is done, so it has potential
security issues (though at least with %rename the input is limited to
valid C/C++ symbol names).
Fixes#1806
