Add support for constant fields and variables

[jaccomoc]

Jactl does not support static class fields in order to not have to deal with thread synchronisation issues and because it is intended to be run distributed applications where the idea of a single class value across multiple instances makes no sense.
Since the problem with static class fields is the fact that they can be changed we could support read-only static fields that are simple types.
This would allow Jactl to provide symbolic names for constants that might be used in several places in scripts and to allow them to be matched against in switch expressions (for example).