Spirit V3.0.4 (Boost V1.71.0)

Explicitly delete all non implemented assignment operators. GH#489 Romain Geissler
Unpublish x3/support/utility/testing.hpp. GH#492
Removed unused is_char and is_string traits. GH#507
Simplified string_traits. GH#507
raw parser attribute materialization issue. GH#509
Removed static assert guarding unwanted attribute materialization that happens when rule parser producing an attribute called without an actual attribute. See GH#511 for the discussion.