In subspan function, add static checks on extents

[mike919192]

Potential fix for issue #842

etl::span subspan function does not currently statically check extents. std::span does check this.

Code demonstration:

std::array<int, 10> array {0};

//std::span std_span1 = std::span(array).subspan<15, 1>(); //compile time error
//std::span std_span2 = std::span(array).subspan<5, 8>(); //compile time error

etl::span etl_span1 = etl::span(array).subspan<15, 1>(); //currently no compile time error
etl::span etl_span2 = etl::span(array).subspan<5, 8>(); //currently no compile time error

This PR should enable c++11 and newer to have compile error when the extent check fails.

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[mike919192]

I also realized that the first and last functions also behave similarly, so I have also added the checks for those functions. I believe those are all the functions where static checks are possible.

std::array<int, 10> array {0};

//std::span std_span1 = std::span(array).first<15>(); //compile time error
//std::span std_span2 = std::span(array).last<15>(); //compile time error

etl::span etl_span1 = etl::span(array).first<15>(); //currently no compile time error
etl::span etl_span2 = etl::span(array).last<15>(); //currently no compile time error

I don't know if it makes sense to also add the static assert for the pre c++11 subspan function? I'll await @jwellbelove feedback.

[jwellbelove]

The ETL_STATIC_ASSERT will work for pre-C++11 as the macro defines a C++03 static assert implementation.

[mike919192]

Thanks. I duplicated the static asserts for the pre c++11 subspan function. I should be finished with changes unless there are other issues.