Fixed incorrect 'valid' flag in assignment operator for arithmetic specialisation

#634-Strange-behavior-in-move-assignment-of-optional

Fix to unique_ptr for polymorphic types

#628 Fixed set of ETL_NO_STL flag in meson.build
#631 unique_ptr derived copy or move to base does not compile
Removed duplicate include in etl::array_view

New features + fixes

#596 Helper functions to convert strings to numeric values. etl::to_arithmetic
#600 Fix 'maybe-uninitialized' g++ error on macros
#601 Move __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS before #include <stdint.h>
#602 Replace ETL_COMPILER_ICCAVR by ETL_COMPILER_IAR
#603 Add back/front insert iterator
#604 Update-test-sources-in-meson-build
#605 Special case check for hashing -0.0
#606 Fix non-usage of key equal function
#610 Added select1st and select2nd
#612 Automatically detect native endianness for IAR compiler platform
#613 Fix bare assert in reference_counted_object.h
#614 unaligned types: use unsigned storage type
#615 Addition of etl::expected (partial). Implementation of etl::unexpected
#616 Provide cmake library for UnitTest++
#618 fix bug in find_next of (new) bitset class
#621 No need to initialize the C-compiler, small speed improvement
#626 Empty etl::optional ctor storage initialization performance
Removed constexpr for etl::bit_cast due to unreliability of compiler support
Added has_value() as an alias for is_value() for etl::result (consistancy with STL conventions)
Added ETL_ERROR_WITH_VALUE macro for exceptions that require a value
Changed scaling template parameter for etl::scaled_rounding to uint32_t
Remove redundant etl::pair functions
Updated string_view and char traits

New features + fixes

Changed explicit message constructor for unsupported message types
Added etl::circular_iterator and circular iterator extensions to etl::span
Added etl::bitset_ext
Added etl::lsb_mask, etl::make_lsb_mask, etl::msb_mask and etl::make_msb_mask to binary utilities
Updates for unordered maps and sets for an API that better matches the STL
Fixes for etl::result<void, TError>
Deleted copy constructors for etl::circular_buffer_ext, etl::indirect_vector_ext and etl::vector_ext to eliminate dangling pointers
Fix non-C++03 code syntax
Fixed unordered container equality tests to match STL
#581 Hash function for enums
#583 Permit non-default-constructable hashes and key-equals
#584 unordered_set::const_iterator has incorrect value_type
#585 Fixed etl::result<void> being unusable due to deleted default constructor
#587 Fixed IAR build warnings in message_packet
#589 Add minmax_push.h/minmax_pop.h includes
#593 Workaround for __has_include(<...>) and ICCAVR
#591 Automatically define __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS

etl::message_broker + constexpr bitset

Added etl::message_broker to message framework
Added 'successor' parameter constructors to all etl::imessage_router derived classes
Added etl::binary_search to algorithm.h
Standardise conditional compilation macros for ETL_USING_CPP11 and ETL_USING_STL
#546: Added a new variant of bitset that may be constexpr
Removed the use of ETL_FORCE_CONSTEXPR_ALGORITHMS as all algorithms may be constexpr
Rationalised etl::send_message functions
Fixed implicit virtual warning for etl::vector
Fixed C++03 delegate compatibility
Added missing char_traits unit tests and char_traits bug fixes
Added default message id constructors
Added ability to set the bitset from wchar_t, char16_t and char32_t
Added ARM5 and ARM6 compiler compatibility for GCC atomics
#579: Fixed Unexpected namespace for variant with non-CPP11 compiler
#580: Fixed numeric_limits redefinition for gnu c++20

Added ability to set the buffer after construction for circular_buffer_ext

Added set_buffer and is_valid member functions.
Added 'size only' constructor.

All warnings resolved for GCC and clang unit tests

#571 Activate GCC and clang compiler warnings
#575 Bip buffer improvements
#576 Invoke function pointers with parenthesis
#574 Allow users to remove SYSTEM keyword because it forces C linkage for some gcc versions

Various fixes

#569 Fixed swap function for circular_buffer_ext
#568 Fixed circular_buffer iterator -> operators
Optimised container move for external buffers for etl::list and etl::forward_list
Added functions and macros to etl::debug_count
Added tests for error handler macros

#567 error-handler: only return when the condition is false

Fixed ETL_ASSERT_XXX macros for ETL_IS_DEBUG_BUILD and !defined(ETL_LOG_ERRORS)

Small fixes to etl::poly_span

Addition of extra ETL_NODISCARD and ETL_NOEXCEPT to etl::span & etl::poly_span.
Fixed warnings for initialisation order for some etl::poly_span constructors.