Updated IO Port API. Various features and fixes

Refactored:
Breaking change.
Modified io_port classes so that they are not derived from iterator types.
Added iterator and const_iterator types.
Added iter() and citer() member functions to get an iterator.
Added &= |= ^= <<= >>= and ~ operators (only available for io_port_rw and io_port_wos types)
Added static asserts to only allow integral types.

Fixes:
#921 unique_ptr reset
#923 Missing equality operator for class expected
#930 Unused parameter warnings
#931 Using etl::make_vector to make a vector of two pairs causes a compile error

Features:
#918 Request for additional operator support for etl::io_port_*
#919 callback_timer: expose callback_timer_data::is_active() as icallback_timer public method
#920 Define ETL_USING_LEGACY_VARIANT as a convenience macro for code that tries to support both new and legacy variants
#940 Allow etl::observer notification without argument

Pull Requests
#905 Add-support-for-type_identity
#909 fix c++20-compat false positive
#914 Fix doc comment for pool::allocate()
#924 Add an etl::nullptr_t type to <etl/nullptr.h>
#925 Add equality operators for class expected
#926 Fix issue #923 equality operator for class expected
#927 Fix missing partial specialisation equal operators
#928 Add missing swap implementations for expected
#929 Fix run-tests.sh script
#932 Fix issue #931 Using etl::make_vector to make a vector of two pairs causes a compile error
#934 Fixes compiler detection with Wundef
#935 Added 20 bit integral limits (MSP430 devices)
New configuration macro, ETL_USE_20BIT_TYPES, which controls the definition of ETL_USING_20BIT_TYPES and ETL_NOT_USING_20BIT_TYPES.
#939 Fix histogram indexing when accessed via operator[]