client: use DEFAULT_CIPHER_SUITES if none are specified

[kevinburke]

I'm having trouble figuring out how to pass in a set of cipher suites
using C, and since all I want is the default set, just make it easier
by providing a reasonable default for the NULL case.

Fixes #163.

[jsha]

As I mentioned in #203 (comment), I'd like a more explicit way to do this:

rustls_client_config_builder_new_custom(
  RUSTLS_DEFAULT_CIPHER_SUITES,
  RUSTLS_DEFAULT_CIPHER_SUITES_LEN,
  tls_versions, // provided by your program
  tls_versions_len,
  &builder);

DEFAULT_CIPHER_SUITES should be defined like so:

const struct *rustls_supported_cipher_suite[] DEFAULT_CIPHER_SUITES;
const size_t DEFAULT_CIPHER_SUITES;

To make this happen we'll need to declare our own copy of DEFAULT_CIPHER_SUITES from rustls, since ours will need to contain pointers to the entries of rustls::DEFAULT_CIPHER_SUITES.

To ensure our copy remains up-to-date, we can have a test that checks the length is the same, and the set of items is the same.

We should document that the order of DEFAULT_CIPHER_SUITES will change between releases.

[jsha]

I think this is resolved by #242