Valhalla repository management

[Karen Kinnear]

The Valhalla repositories have been restructured and rebased on jdk10/hs so
as to remain current with the latest post-JDK9 changes.

We have a new model of creating sandboxes with branches, designed to
enable us to ship features more quickly, to bind features to releases later and
to automagically keep repositories current with main line changes.

An overview of the sandbox and branch approach is described in [1]. We are
working to improve and simplify these instructions as we speak. It is my understanding
that once you have created my_local_branch, and are operating inside the local branch,
that the usual hg commands will work and assume the current branch.

The new Valhalla repository structure looks like this [2]
  valhalla/valhalla - jdk10/hs based 
  valhalla/valhalla9 - read-only saved version from jdk9 timeframe prototyping

Under valhalla/valhalla the branches look like this [3]
  initial branches include:

mvt - for Minimal Value Types prototyping
default - do NOT push to this repository - this is used to automagically update from jdk10/hs changes
nestmates

The mvt repository has just been updated to contain an extracted subset of the minimal value types
work that was in the earlier valhalla repository. This is where we will continue to make changes.

I expect the nest mates repository to not be updated quite yet - we are in the midst of some
complex discussions on JVMS invoke special verification handling.

thanks,
Karen

[1] http://cr.openjdk.java.net/%7Echegar/docs/sandbox.html
[2] http://hg.openjdk.java.net/valhalla
[3] http://hg.openjdk.java.net/valhalla/valhalla/branches