Add checks for extent compatability in span range constructor

[libbooze]

Based on discussion in #179 / @pdimov suggestion.

few notes

1. This will break some current code that does not exibit UB, for example constructing span of extent 3 fromboost::arrayof size 4. As this is not allowed for std::array I presumed same logic should follow for other ranges.
2. I did not know a nicer C++11 way to produce an error message if extents do not match, so I used this mechanism with conversion of argument to resolve ambiguity
3. regarding formatting: I found no clang-format file in repo so I did my best to do what seems right
4. as for tests: I was trying to follow existing practices
5. lmk if comments are too extensive
6. if span_default_constructed_size works correctly maybe it should be moved to separate header if we think it will be useful in more places
7. I did not add runtime checks with BOOST_ASSERT, I prefer to do that in separate PR
8. I did not add checks for max_size, that may be useful for some fixed buffer types, e.g. static_string
9. for std::span availability in test I used C++23 macro(I presume all C++23 compilers have all C++20 headers) since it is easier than to check for span header, if you prefer I can switch to specific span check
10. I used boost::array as a nice range type, I know core can not depend on almost anything, but I presumed it is fine in tests a I think boost array does not depend on core, if not let me know so I make a simple test type.
11. b2 cxxstd=03 test fails, not sure if that is fine as C++11 is new minimal Boost version

[glenfe]

I'm not OK with this change. I'll provide rationale when I have some time.