boost: Package Version Update (1.73.0)

This commit updates Boost to version 1.73.0
In this release, there are two new libraries
 - Nowide [2] - Standard library functions with UTF-8 API on Windows,
   from Artyom Beilis.
 - Static String [3] - A dynamically resizable string of characters
   with compile-time fixed capacity and contiguous embedded storage,
   from Vinnie Falco and Krystian Stasiowski.

More info about Boost 1.73.0 can be found at the usual place [1].

[1]: https://www.boost.org/users/history/version_1_73_0.html
[2]: https://www.boost.org/libs/nowide/
[3]: https://www.boost.org/libs/static_string/

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>

libs/boost/patches/002-outcome-assert.patch

@@ -0,0 +1,11 @@
+--- a/boost/outcome/experimental/status-code/status_code_ptr.hpp     2020-04-28 09:23:14.792565200 +0100
++++ b/boost/outcome/experimental/status-code/status_code_ptr.hpp     2020-04-28 09:24:34.311741000 +0100
+@@ -103,7 +103,7 @@
+ #endif
+     virtual void _do_erased_copy(status_code<void> &dst, const status_code<void> &src, size_t /*unused*/) const override  // NOLINT
+     {
+-      assert(dst.domain() == *this);
++      // Note that dst will not have its domain set
+       assert(src.domain() == *this);
+       auto &d = static_cast<_mycode &>(dst);               // NOLINT
+       const auto &_s = static_cast<const _mycode &>(src);  // NOLINT