-Add: 'virtual' packets (to be usable as fields as well).
-Change: rename PacketFieldBase to FieldBase.
-Add: force_virtual meta opt to force all childs to be virtual by default as well.

Useful for packetfields.
-Change: add ordering to the fields, and remove is_struct_field.

the latter will be done differently.
-Change: add can_merge and merge functions to FieldBase.
-Add: can_merge logic to StructField

I.. somehow managed to commit the StructField changes earlier without
mentioning.. bad Xaroth!
-Fix: ordering of Fields.

Also removed a superfluous print
-Change: re-order imports.
-Add: import StructField into the main list as well.
-Add: Merge logic for fields.