Make span std compliant (#317)
* add missing overloads for span::first + span::last
The C++20 standard defines additional overloads for first and last:
template< std::size_t Count >
constexpr std::span<element_type, Count> first() const;
constexpr std::span<element_type, std::dynamic_extent> first( size_type Count ) const;
template< std::size_t Count >
constexpr std::span<element_type, Count> last() const;
constexpr std::span<element_type, std::dynamic_extent> last( size_type Count ) const;
etl implements only the first (= template) variants so far. To be able to
compile valid C++20 code the missing overload should be added.
* remove explicit specifier for span conversion operator
The C++20 standard allows to assign a span of non-const elements to a span of
const elements. Example:
std::span<const int> cintspan;
std::span<int> intspan;
cintspan = intspan;
This is enabled in the STL by using an explicit specifier with a constant
expression for one of the conversion constructors:
template< class R >
explicit(extent != std::dynamic_extent)
constexpr span( R&& r );
The explicit specifier together with a constant expression is a C++20 feature
and therefore can't be used within etl. To be able to compile valid C++20
code which uses the conversion on assignment, the explicit specifier has to
be removed.
* remove explicit specifier for span conversion operator
The C++20 standard allows to assign an array of elements directly (without
explicitly using a conversion constructor). Example:
const int data = { 1, 2, 3 };
std::span<const int> cintspan;
cintspan = data;
To be able to compile valid C++20 code which uses the conversion on assignment,
the explicit specifier of the array-conversion constructor has to be removed.
