Avoid polymorphic comparison functions in OpamListCommand #4068

Job	Run time
Analyse	4s
Test-macOS (4.14.2)	27m 10s
Build-Linux (4.08.1)	2m 26s
Test-macOS (5.3.0)	27m 30s
Build-macOS (4.14.2)	3m 34s
Build-Linux (4.09.1)	2m 41s
Build-macOS (5.3.0)	2m 52s
Build-Linux (4.10.2)	3m 17s
Cygwin	5s
Build-Linux (4.11.2)	2m 28s
Hygiene	15s
Build-Linux (4.12.1)	2m 30s
Build-Linux (4.13.1)	3m 15s
Build-Linux (5.0.0)	2m 45s
Build-Linux (5.1.1)	2m 17s
Build-Linux (5.2.1)	2m 25s
Build-Linux (4.14.2)	2m 16s
Build-Linux (5.3.0)	3m 16s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	35m 34s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 30s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 32s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	45m 28s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	46m 6s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.3.0)	40m 56s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 5.3.0)	41m 28s
Cold-Linux	5m 16s
Doc-Linux (4.14.2)	1m 6s
Test-Linux (4.14.2)	11m 17s
Solvers-Linux (z3, 4.14.2)	2m 20s
Doc-Linux (5.3.0)	1m 12s
Test-Linux (5.3.0)	11m 0s
Solvers-Linux (z3, 5.3.0)	56s
Upgrade-Linux (4.14.2)	2m 34s
Solvers-Linux (0install, 4.14.2)	1m 25s
Upgrade-Linux (5.3.0)	1m 15s
Solvers-Linux (0install, 5.3.0)	1m 38s
Upgrade-macOS (4.14.2)	1m 46s
Solvers-macOS (z3, 4.14.2)	1m 7s
Upgrade-macOS (5.3.0)	2m 23s
Solvers-macOS (z3, 5.3.0)	1m 45s
Solvers-macOS (0install, 4.14.2)	1m 10s
Solvers-macOS (0install, 5.3.0)	1m 13s
	7h 13m 3s

Provide feedback