[foreign-memaccess] RFR 8228447: Remove kinds from value layouts

[Maurizio Cimadamore]

Hi,
this patch removes the Kind enum from ValueLayout - and consolidates the 
various ValueLayout factories even further:

cr.openjdk.java.net/~mcimadamore/panama/8228447/

(this is based on top of the patch for 8228444).

I like how it simplifies the API, and removes the 
float/int/signed/unsigned from the equation which, as Jorn pointed out, 
wasn't  really used anywhere.

Maurizio