Use std::unique_ptr to guard against leaking the new'd pointers
in SEXPR lists if there is an exception during parse.
This was noticable during the Sexpr/ParseExceptions unit test.
Fixes: lp:1830209
* https://bugs.launchpad.net/kicad/+bug/1830209
The SEXPR class is a useful general-purpose S-Expression library
class and can (maybe) be used else where. It also should get
test coverage, as even if noone else uses it, it's critical for the
kicad2step exporter.
Also add some test coverage for some kicad2step routines. For now,
they're not useful outside kicad2step, but they are at least a useful
reference for S-Expression parsing.
Jeff Young
Seth Hillbrand
John Beard