Code Review Request : signed module digest verification

[Sean Mullan]

See webrev: 
http://cr.openjdk.java.net/~mullan/jigsaw/webrevs/digest-verification/webrev.00/

With this change, signed module [1,2] digests are now verified as part of the 
signed module installation and verification. This does not address the 
ModuleFileFormat.Reader issues I previously raised [3]; those will be addressed 
in a subsequent webrev.

Thanks,
Sean

[1]: http://cr.openjdk.java.net/~mullan/jigsaw/signed-module-file-format
[2]: http://cr.openjdk.java.net/~mullan/jigsaw/signed-module-functional-spec
[3]: http://mail.openjdk.java.net/pipermail/jigsaw-dev/2011-March/001185.html