Added polymorphic span

Added etl::poly_span, a std::span concept for polymorphic object types.
Refactored parts of etl::span, including prevent compound statements in constexpr methods for C++11
Added etl::is_enum to type_traits.h

Fixed #560 Unable to upcast legacy variant

Refactored legacy variant to remove upcast functors.
Added upcast_ptr, is_base_of & 'not a base' error exception to legacy variant.
If ETL_USE_LEGACY_VARIANT is defined then legacy variant is in the etl namespace.
If ETL_USE_LEGACY_VARIANT is not defined then legacy variant is in the etl::legacy namespace.
Added non-member etl::send_message for etl::shared_message
Green Hills compiler compatibility

Minor update

Updated message_packet_generator.h to match etl::message_packet changes

Fix to etl::message_packet

#557 QueuedFSM example does not compile
Updated atomic tests for GCC compatibility.

Added etl::byte_stream_overflow assert to byte_stream_reader 'skip'.

20.29.1

dded etl::byte_stream_overflow assert to byte_stream_reader 'skip'.

Added bit_stream_reader, bit_stream_writer + other updates

Added etl::bit_stream_writer
Added etl::bit_stream_reader
Deprecated etl::bit_stream
Added callback option to etl::byte_stream_writer
Added error exceptions to byte_stream_writer
Added ETL_NODISCARD to etl::delegate create and is_valid functions
Added etl::visit support to etl::variant
Refactored C++17 message_packet
Refactored etl::atomic implementations to allow non-(integrals/pointers/bool)
Refactors etl::vector and etl::deque resize() to take const reference parameter
Renamed ETL_ALWAYS_ASSERT to ETL_ASSERT_FAIL
Removed duplicate void_t definition
Removed duplicate etl::declvar definition
Renamed cumulative_moving_average to pseudo_windowed_moving_average to more accurately reflect its algorithm.
Changed etl::debounce internal state names to avoid clashes with Arduino macros.

Fixed issues raised by sanitizer + other updates/fixes

Fixed issues raised by sanitizer.
Added conditional compilation for 8 bit type in hash.h
Fixed warnings for 64bit compilation.
Fixed sanity check includes.
Fixed incorrect returned span length for byte stream read.
Updates to etl::successor and derived classes.

Fixed compilation error of etl::reference_counted_message_pool with ETL_LOG_ERROR enabled

#531 Fixed compilation error of etl::reference_counted_message_pool with ETL_LOG_ERROR enabled, due to non-public inheritance of base exception class.

Various fixes + additional traits + optimisations

Added scripts to automatically update version information.
Added ETL version information to etl::traits.
Small optimisations for computing indexes for queues & cyclic_value.
#530 Fixed: etl::nth_type not implemented correctly.
#521 CMake: Add package version file for installation.
#525 Fixed: cplusplus constant type.
#517 Fixed: unused-local-typedefs warning.

Fixed undefined macros when compiler type is defined by the user

Updated version numbers