RFR: 8171373: Reduce copying during initialization of ModuleHashes

[Claes Redestad]

Hi,

recent changes to split out ModuleHashes from ModuleDescriptor caused a 
small/tiny increase in HashMap creation, resize and copying:

Webrev: http://cr.openjdk.java.net/~redestad/8171373/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8171373

This decreases #bytecode executed during startup by ~20K (around 1.5% of 
total).

Thanks!

/Claes