Do not create %FIELDS if it doesn't exists

Checking the existence of %FIELDS was creating it. This could trigger
'once' warnings on (at least) perl 5.10.1.
Constify $^V check to allow inlining

Allows perl to optimize the generated code.
Do not load fields.pm and Hash::Util

When we use them (in T::MO::Extends), this is to extends a
fields-based class. In that case, the original class has already loaded
those modules.
Merge pull request #5 from dolmen/fix/FIELDS-once

Fix injected %FIELDS by T::MO::Extends