[foreign-memaccess] RFR 8228444: Add common value layout constants

[Maurizio Cimadamore]

Hi,
this is a first stab at implementing a set of layout constants along the 
guidelines described in [1].

http://cr.openjdk.java.net/~mcimadamore/panama/8228444/

More specifically, the Layouts class introduces layouts constants to be 
used for language agnostic serialization, with explicit endianness:

BYTE_LE, BYTE_BE, BYTE_2_LE, BYTE_2_BE, ...

And some Java-friendly constants whose endianness defaults to BE (same 
as ByteBuffer):

JAVA_BYTE, JAVA_SHORT, ...

In the future, we will create nested interfaces here, one per supported 
ABI, with ABI-specific layout constants - e.g.

SysVABI.C_INT, SysVABI.C_LONG_DOUBLE, ...

This patch also removes endianness implicit constructors from 
ValueLayout - now endianness is always explicit (but if you use 
constants, you can ignore it, up to a point).

Overall this seems like the right mix of decisions - different 
constants, different users, different requirements. Not sure I totally 
love the names, the challenge here is to find something that is relative 
semantics-neutral (since we plan to 'free' layouts from semantics 
considerations such as signed/unsigned int/float).


[1] - 
https://mail.openjdk.java.net/pipermail/panama-dev/2019-July/006001.html