Sync to 20.32.1

20.30.0

• 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

20.30.1

• Fixed #560 Unable to upcast legacy variant Latest
• 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

20.31.0

• Added etl::poly_span
• Refacted parts of etl::span, including prevent compound statements in constexpr methods for C++11
• Added etl::is_enum

20.31.1

• 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.

20.31.2

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

20.31.3

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

20.32.0

• #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

20.32.1

• Added size only constructor, set_buffer() and is_valid() members to circular_buffer_ext