[foreign-memaccess] RFR 8228487: Find a better name for MemoryLayout::ofValue

[Maurizio Cimadamore]

Hi,
this is a simple (but probably controversial) patch. I realized that 
when we consolidated all the factories for ValueLayout we were left with 
a very general-sounding:

MemoryLayout.ofValue(32, ....)

This kind of gives the feeling that 32 is the _value_, not the size in bits.

This patch changes that to 'ofValueBits' and does a similar change 
ofPadding -> ofPaddingBits

(leaving the door open to ofValueBytes and ofPaddingBytes).

I've also considered MemoryLayout.ofBits as an alternative, but found it 
too vague (after all, even padding is "bits").

I think I like ofValueBits/ofPaddingBits, but these factory names are 
now slightly inconsistent with the constant names in MemoryLayouts - can 
we live with that? Or do we need better constant names too?

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8228487/

Maurizio