Allow catch behavior to be customized

[dtolnay]

As proposed in #53 (comment). If the user's cxx::bridge invocation includes any header that defines the following function, they get it's behavior as the exception⟶Result conversion.

namespace rust::behavior {
template <typename Try, typename Fail>
static void trycatch(Try &&func, Fail &&fail) noexcept try {
  func();
} catch (/* up to you */) {
  fail(/* const char *msg */);
}
}

The default behavior is equivalent to:

} catch (const std::exception &e) {
  fail(e.what());
}

Codebases that use Folly, for example, may be interested in behavior like this instead for better type information on the error messages:

} catch (const std::exception &e) {
  fail(folly::exceptionStr(e));
} catch (...) {
  fail("<unknown exception>");
}

[dtolnay]

Apparently doesn't work in x86_64-pc-windows-msvc.

C:\Users\travis\build\dtolnay\cxx\target\debug\build\cxx-test-suite-2db38eca0276da33\out\tests\ffi\lib.rs.cc(190):
error C2904: 'trycatch': name already used for a template in the current scope