ffi: add functions to deal with sliced strings (fix #1909)

[ichordev]

Adds *_sliced variants of the following functions that accept an additional length parameter for strings instead of using null-termination:

• quiche_version_sliced
• quiche_enable_debug_logging_sliced
• quiche_config_load_cert_chain_from_pem_file_sliced
• quiche_config_load_priv_key_from_pem_file_sliced
• quiche_config_load_verify_locations_from_file_sliced
• quiche_config_load_verify_locations_from_directory_sliced
• quiche_config_set_cc_algorithm_name_sliced
• quiche_connect_sliced
• quiche_conn_set_keylog_path_sliced
• quiche_conn_set_qlog_path_sliced
• quiche_conn_set_qlog_fd_sliced

This fixes issue #1909.
I'm not very experienced with Rust, so please double-check that I didn't do anything wrong in ffi.rs. I'm using std::str::from_utf8_unchecked to create the str& in Rust so that there's no runtime overhead for checking UTF-8 validity.

[ichordev]

Is this check failing because of mis-formatting?