Versions after v1.15.6 disable access to `vm.Program.Location`

[buixor]

ref: #544

with v1.15.6, some fields of vm.Program aren't accessible/public anymore, such as Locations.

I used to rely on the Locations field to fetch the source code line number associated with a given instruction pointer. By using vm.Step(), vm.Position() and the Locations of vm.Program, it allows to display debug/code to users when debugging their expression in crowdsec.

Making this field public again would allow to continue to provide such a feature!

[antonmedv]

Added program.Locations()