Don't include <cassert>. (#2148)

[toojays]

This commit replaces use of the assert() macro in format-inl.h with
FMT_ASSERT(). This allows us to drop the cassert include.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

Note that the change to FMT_ASSERT in test-assert.h is to avoid the following warning:

In file included from /home/jscott/src/fmt/test/format-impl-test.cc:10:
/home/jscott/src/fmt/include/fmt/format-inl.h: In destructor ‘fmt::v7::detail::bigint::~bigint()’:
/home/jscott/src/fmt/test/test-assert.h:26:52: warning: throw will always call terminate() [-Wterminate]
   26 |   if (!(condition)) throw assertion_failure(message);
      |                                                    ^
/home/jscott/src/fmt/include/fmt/format-inl.h:1300:15: note: in expansion of macro ‘FMT_ASSERT’
 1300 |   ~bigint() { FMT_ASSERT(bigits_.capacity() <= bigits_capacity, ""); }
      |               ^~~~~~~~~~
/home/jscott/src/fmt/test/test-assert.h:26:52: note: in C++11 destructors default to noexcept
   26 |   if (!(condition)) throw assertion_failure(message);
      |                                                    ^
/home/jscott/src/fmt/include/fmt/format-inl.h:1300:15: note: in expansion of macro ‘FMT_ASSERT’
 1300 |   ~bigint() { FMT_ASSERT(bigits_.capacity() <= bigits_capacity, ""); }
      |               ^~~~~~~~~~

Actually it seems that just moving the throw into a separate function (even inline) is enough to squelch the warning, with GCC 9.3 at least. In the end I went with this more explicit version.

[vitaut]

Thanks for the PR.