RFR(s): 8156079: make empty instances singletons

[Stuart Marks]

Hi all,

Please review this small set of changes to the immutable collections to make the 
zero-size instances singletons. (JDK-8156079)

This also includes a couple other small optimizations (JDK-8169222) to change 
the expansion factor from floating point to integer, and also to avoid creating 
a single-use Random instance to compute the hash salt value at startup.

Webrev:

     http://cr.openjdk.java.net/~smarks/reviews/8156079/webrev.0/

Thanks,

s'marks