Deprecate instead of remove index_type

mstd::span rename index_type to size_type according to spec

Fix make_span text in comment block

Fix last extra space issues

Backport dynamic extent when including <span> ...

Keep make_span functions even when <span> is available for compatibility

Fix missing defintion

Add some comment blocks

Add deduction guides

Remove nonstandard as_bytes

Fix minor style issues

Replace classes with structs where applicable

Use original <span> if present and the right version

Missed a pointer operator

Fix pointer and reference operator style

Fix newline at end of file

Update default initializers

Remove explicit constructors

Fix remaining namespaces and add include files

Remove c++17 inline of constexpr

Fix namespaces and span prototype

Prevent warning on subspan<0,N>

Add container constructor and make_span

Correct span constructors

Fix endif comment

Adds basic span implementation