RFR: 8237878: Improve ModuleLoaderMap datastructures

[Claes Redestad]

Hi,

refactor ModuleLoaderMap to allow the datastructure to be
archived, then archive it.

Webrev: http://cr.openjdk.java.net/~redestad/8237878/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8237878

Testing: tier1-3

/Claes