[msgs::ChannelReestablish] implement encoding and decoding

[msgs::ChannelReestablish] implement encoding and decoding

Summary:

Implementing these, taking inspiration from the surrounding code since i'm a rust n00b.

Test Plan:
```
cargo build
cargo test channel_reestablish
```
output:
```
running 3 tests
test ln::msgs::tests::encoding_channel_reestablish_with_secret ... ok
test ln::msgs::tests::decode_channel_reestablish_bad_length ... ok
test ln::msgs::tests::encoding_channel_reestablish_no_secret ... ok

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 14 filtered out
```