Added WeakHashSet and related test files. A WeakHashSet weakly
holds its items, so when the items are no longer strongly referenced, they are collected and automatically removed from the set.
A src/edu/rice/cs/plt/collect/WeakHashSetTest.java A src/edu/rice/cs/plt/collect/WeakHashSet.java
Authored by: c45207 2007-01-26
Parent: [r4093]
Child: [r4095]
