clang fail to build without this static_cast

cast int const to alphabet_index_t type

This changes the include path from <catch.hpp> to <catch2/catch.hpp>,
and gets rid of all remaining warnings on VS2017.
(VS2015 still emits a bunch of questionable warnings.)

Update Catch to v2.3.0.

<Windows.h> includes macro definitions named max and min, wrap the call to max and min with parentheses, which prevent the macro expansion

Fix tplgy#62 - compile error when including <Windows.h> due to macro expansion

This is "Base 32 Encoding with Extended Hex Alphabet"

Fix comment referring to the wrong RFC encoding

By taking the padding character put call out of the tail() function
and appending it after the tail() call in encode(), the compiler
will work with a more limited set of numbers known at runtime
instead of a slightly longer list of consecutive numbers at
compile time. The code paths that would cause an unreachable code
warning are now actually removed and would also be impossible
to tell in advance by the compiler.

Code sizes remain the same for clang as well as gcc in all
but debug builds, where they only marginally differ.

What's weird is that this change makes my GCC 8.2 release build
perform better on the "Decoding to vector<uint8_t>" benchmark -
weird because none of the changed code is called by any decode()
code paths. But hey, I'll take it. Average time for this task
go down from approx. 67 microseconds for the size 32768 buffer
to approx. 54 microseconds on my machine. Clang appears unaffected.

This should make sure it doesn't suffer from unnecessary slowness.
The good news is that GCC and Clang release builds don't seem to
be noticeably affected by this change, they must have already
done the right thing (inlining) beforehand. Nonetheless, compilers
with less predictive skill may get a speed-up from this.

Also, remove raw_result_buffer::last() because it's not needed
and makes it look like it's necessary for data wrappers, which
it's not.

Fix a recent warning and ensure inlining for less-smart compilers.

This makes CMakeLists.txt suitable for inclusion into a
parent CMake project via add_subdirectory() command,
and defines the "cppcodec" target as INTERFACE library.
Use it in a target_link_libraries() command as dependency
in order to add cppcodec's include directories to your target.

The minimum CMake requirement is now 3.13, picking up various
improvements to INTERFACE library targets and avoiding any
confusion around CMake policy CMP0076, a behavior changed in 3.13.

The CMake build will now build tests, examples and tools if built
stand-alone, but exclude them if built as part of another project.
This can be customized via CPPCODEC_BUILD_{TESTING,EXAMPLES,TOOLS}
CMake definitions for a given build.

Fixes tplgy#60
Fixes tplgy#61

This commit is based on changes by Quintz Gábor (@quiga on GitHub),
but modified and extended in a number of ways.

Thanks also to Aditya Gupta (@adi-g15), anonymous @ghuser404
and Kingsley Chen (@kingsamchen) for keeping this issue on
my radar and providing valuable feedback.

Allow using cppcodec as a CMake dependency