Fixed issues #948 & #949

#948 Bug in queue pop can break queue.empty()
Also included etl::stack in the fix.
#949 For GCC10, we should check __cplusplus >= 201709L when determining if c++20 is supported

Fix unaligned_type for floats + various small fixes

Updates:
Removed C++11 restriction on etl::observer::notification
Removed EmbeddedTemplateLibrary-20.39.0.tar.gz from project.

Fixes:
#917 unaligned_type doesn't compile with floating point types

Pull Requests:
#946 Make include paths to private files relative

Removed EmbeddedTemplateLibrary-20.39.0.tar.gz from project.

EmbeddedTemplateLibrary-20.39.0.tar.gz included in error.

Fixes and new type traits

Fixes:
#940 Allow etl::observer notification without argument
Added a void specialisation to the < C++11 code.
#942 Warning on pragma c++20-compat

Updates:
Added etl::count_of, etl::has_duplicates, etl::has_duplicates_of

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[]

#895 Removed std algorithm calls from algorithm.h

Missing changes from last release

Various fixes

Fixes:
#889 bitset_new.h throws warnings for GHS compiler
Fixed ordering of static & ETL_CONSTEXPR14.
#895 Removed std algorithm calls from algorithm.h

Changes without a GitHub issue#:
Added extra tests for as_bytes and as_writable_bytes
Made conversion operator for C++03 etl::byte public

Removed ETL_CONSTEXPR for string and vector size()

#892 Compilation Error with constexpr in etl::string_base::size() on ESP-WROVER-KIT (c++11)
Removed ETL_CONSTEXPR from etl::string_base::size().
Removed ETL_CONSTEXPR from etl::vector::size().

Added GCD, LCM. Iterator specialisations for etl::rotate. Constexpr for etl::absolute

Features:
Added GCD and LCM algorithms. etl::gcd and etl::lcm
Added specialisations of etl::rotate for different iterator types
Added ETL_NODISCARD, ETL_CONSTEXPR14 and ETL_NOEXCEPT to etl::absolute and etl::absolute_unsigned

Revert #804

Revert change #804 to restore value initialisation of type0 for default constructor