Improve the structs in the API, add docs

[LVala]

This PR:

• removes most of the stuff that should be private from RTPTransceiver, RTPSender and RTPReceiver structs (like the "recorder..." modules)
• adds some getters to the PeerConnection (for the various states)
• adds doc stubs to most of the function in the API in general

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.09302% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 86.46%. Comparing base (b158659) to head (b5cb05e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #104 +/- ## ========================================== - Coverage 87.20% 86.46% -0.75% ========================================== Files 33 33 Lines 1626 1655 +29 ========================================== + Hits 1418 1431 +13 - Misses 208 224 +16 ``` | [Files](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) | Coverage Δ | | |---|---|---| | [lib/ex\_webrtc/rtp\_receiver/nack\_generator.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_receiver%2Fnack_generator.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfcmVjZWl2ZXIvbmFja19nZW5lcmF0b3IuZXg=) | `100.00% <ø> (ø)` | | | [lib/ex\_webrtc/rtp\_receiver/report\_recorder.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_receiver%2Freport_recorder.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfcmVjZWl2ZXIvcmVwb3J0X3JlY29yZGVyLmV4) | `96.66% <ø> (ø)` | | | [lib/ex\_webrtc/rtp\_sender.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_sender.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfc2VuZGVyLmV4) | `92.30% <100.00%> (+0.20%)` | :arrow_up: | | [lib/ex\_webrtc/rtp\_sender/nack\_responder.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_sender%2Fnack_responder.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfc2VuZGVyL25hY2tfcmVzcG9uZGVyLmV4) | `100.00% <ø> (ø)` | | | [lib/ex\_webrtc/rtp\_sender/report\_recorder.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_sender%2Freport_recorder.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfc2VuZGVyL3JlcG9ydF9yZWNvcmRlci5leA==) | `96.96% <ø> (ø)` | | | [lib/ex\_webrtc/rtp\_receiver.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_receiver.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfcmVjZWl2ZXIuZXg=) | `74.19% <60.00%> (ø)` | | | [lib/ex\_webrtc/rtp\_transceiver.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Frtp_transceiver.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9ydHBfdHJhbnNjZWl2ZXIuZXg=) | `90.30% <92.30%> (+0.49%)` | :arrow_up: | | [lib/ex\_webrtc/peer\_connection.ex](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?src=pr&el=tree&filepath=lib%2Fex_webrtc%2Fpeer_connection.ex&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc#diff-bGliL2V4X3dlYnJ0Yy9wZWVyX2Nvbm5lY3Rpb24uZXg=) | `80.43% <62.26%> (-2.10%)` | :arrow_down: | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Last update [b158659...b5cb05e](https://app.codecov.io/gh/elixir-webrtc/ex_webrtc/pull/104?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=elixir-webrtc).