Adding missing span deduction guides

[JordanMaples]

I noticed while experimenting with std::span that some of the functionality wasn't 1:1 regarding CTAD.
The deduction of container types was missing. The following std::span example compiles without any problems, while the the gsl equivalent does not.

Ex: Current behavior

std::vector v{1,2,3,4}; 
std::span sp{v}; // no problem, compiles

std::vector v{1,2,3,4}; 
gsl::span sp{v}; // fails to compile. gsl::span<int>, however does compile

By examining the container's value_type in the deduction guide, we're able to deduce the internal type and construct the span. This more closely aligns gsl::span's behavior to std::span.

[CaseyCarter]

If you only care about containers, this is fine. Non-containers sometimes have const-qualified elements even when they are themselves not const, which will break this. For example, in:

#include <gsl/span>
#include <string_view>
#include <type_traits>

using namespace std::literals;

int main() {
    auto s = "Hello, World!"sv;
    gsl::span x{s}; // Line 9
    static_assert(std::is_same_v<decltype(x)::element_type, const char>);
}

CTAD deduces span<char> on line 9, resulting in the compiler complaining that initialization from const char* is ill-formed.

I suggest instead:

Suggested change

	template <class Container>
	span(Container&)->span<typename Container::value_type>;
	template <class Container>
	span(const Container&)->span<const typename Container::value_type>;
	template <class Container,
	class Element = std::remove_pointer_t<decltype(std::declval<Container&>().data())>>
	span(Container&)->span<Element>;
	template <class Container,
	class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>>
	span(const Container&)->span<Element>;

[CaseyCarter]

Okay, my OCD made me test these, so I'll submit a PR (JordanMaples#6) with this change and the updated test.