[foreign-memaccess] RFR 8226519: Add layout helper methods to obtain size and alignment in bytes

[Maurizio Cimadamore]

Subject says it all: let's get rid of all "/ 8" in the API/tests by 
offering handy (and safe) helpers which do the conversion.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8226519/

I've also slightly renamed the alignment method - to make it consistent 
with the size one.

Cheers
Maurizio