embind should use the length that std::string already knows

Per [a comment on another issue](https://github.com/emscripten-core/emscripten/pull/13304#issuecomment-778637859) embind doesn't make use of `std::string` knowing its length when converting to a JS string. Treating `std::string` as a C string is both unnecessarily inefficient and, in the case of embedded U+0000, in correct.

Please use the length already known and avoid `strlen` when converting a `std::string` to a JS string.