Improve Naming of Evnrionment Slots

[iwillspeak]

Current environment slot naming is of the form _env_0 where 0 is the slot index. It would be nice if this better represented the value that is to be stored there e.g. if captured from an argument it could be _arg_0 or _local_1 for locals. Even better if the field name could include the original name for the field, although this may require some tweaks to how StorageRef is represented to flow that information through to the emit phase.

[iwillspeak]

See also the namimg discussions in #36