support fields::new() and fields::phash() to create pseudo-hash

support fields::new() and fields::phash() to create pseudo-hash
objects and plain pseudo-hashes respectively (this avoids users
from having to diddle %FIELDS directly); update documentation to
suit (from original fields::phash() implementation by Peter
Scott <Peter@PSDT.com>)

p4raw-id: //depot/perl@5293