Spirit V2.5.5

There was no version bump and changelog for Boost V1.61.0-1.66.0 releases.

Fixed keyword directives compilation on C++11 PR#256 #11493.
Fixed subrules compilation PR#279 PR#259.
Fixed subrules use after scope bug PR#284 #5270.
Fixed undefined behavior in sequential or operator. Underlying parsers order of execution was dependent on compiler PR#310.
Spirit was accidentally relaying on types typedef of boost::optional. The typedef was removed in the new optional implementation PR#271 #12349.
Fixed problems with BOOST_SPIRIT_NO_PREDEFINED_TERMINALS (attr_cast, confix, distinct, keywords, and flush_multi_pass) PR#314 #13311.
Added missing copy assignment operator to hold_any PR#361 #8268.

Fixed alternate operator PR#201 PR#271 #12349.
Fixed signed integer overflow in real parser #245.
Fixed overflow problem in uint_parser<signed T> PR#297.
Fixed get_current_line, get_line_start, and added get_line_end PR#304.
Permutations parser were always initializing optionals PR#319 #12473.
Changed iterator concept static assert from ForwardIterator to ReadableIteratorConcept && ForwardTraversalConcept PR#320.
Added assertations to real parsers to ensure that the parse_frac_n from user defined real policy does not return negative values and also to prevent static analyzers false-positives PR#358.

Fixed alternate operator PR#271.
Fixed no_buffering_policy missing copy constructor PR#261.
The ostream_iterator was inheriting std::iterator which is deprecated in C++17 PR#345.

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