Implement data and size functions

doc/data.qbk

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+[/
+Copyright 2023 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+]
+
+[section:data data]
+
+[simplesect Authors]
+
+* Glen Fernandes
+
+[endsimplesect]
+
+[section Overview]
+
+The header <boost/core/data.hpp> provides function templates `data`
+to obtain the pointer to the first element in a range.
+
+[endsect]
+
+[section Reference]
+
+```
+namespace boost {
+
+template<class C>
+constexpr auto
+data(C& c) noexcept(noexcept(c.data())) -> decltype(c.data());
+
+template<class C>
+constexpr auto
+data(const C& c) noexcept(noexcept(c.data())) -> decltype(c.data());
+
+template<class T, std::size_t N>
+constexpr T*
+data(T(&a)[N]) noexcept;
+
+template<class T>
+constexpr const T*
+data(std::initializer_list<T> l) noexcept;
+
+} /* boost */
+```
+
+[section Functions]
+
+[variablelist
+[[`template<class C> constexpr auto data(C& c) noexcept(noexcept(c.data())) ->
+decltype(c.data());`]
+[Returns `c.begin()`.]]
+[[`template<class C> constexpr auto data(const C& c)
+noexcept(noexcept(c.data())) -> decltype(c.data());`]
+[Returns `c.begin()`.]]
+[[`template<class T, std::size_t N> constexpr T* data(T(&a)[N]) noexcept;`]
+[Returns `a`.]]
+[[`template<class T> constexpr const T* data(std::initializer_list<T> l)
+noexcept;`]
+[Returns `l.begin()`.]]]
+
+[endsect]
+
+[endsect]
+
+[endsect]