This resolves a spec-compliance bug with BOLT 4 where we simply
failed to deserialize the message and thus could never return an
HTLC failure message. However, note that BOLT 4 incorrectly hints
that a non-malformed message should be used ("...MUST report a
route failure to the origin node") which we cannot do as we cannot
derive a SharedSecret to encrypt a regular update_fail_htlc message
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
```
