Generate explicit export list for *_Fields module

[jcmartin]

Right now, the *_Fields.hs modules do not have an explicit export list. This results in a warning when -Wmissing-export-lists flag is enabled. This flag is presently enabled by default on new installs of stack. A quick and dirty approach is to do module A (module A) where in the cases where every function in the module should be exposed. However, an explicit list at the header is more helpful even just for the purposes of being able to see every function in as few of lines as possible.