Export various fields and structures

[devrandom]

This is for discussion purposes at this time. This is a set of patches we maintain on top of master in order to let Rust Lightning Signer do various things, including:

• access to TxCreationKeys fields and constructor, so that we can use it to call various InMemoryChannelKeys methods
• access to LocalCommitmentTransaction constructor for InMemoryChannelKeys use
• export the unsafe methods for testing use, gated by a new feature named "unsafe"
• access to node and channel announcement fields so that the contents can be checked by the signer

[codecov]

Codecov Report

Merging #662 into master will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #662      +/-   ##
==========================================
- Coverage   91.36%   91.35%   -0.01%     
==========================================
  Files          35       35              
  Lines       21681    21688       +7     
==========================================
+ Hits        19808    19813       +5     
- Misses       1873     1875       +2     

Impacted Files	Coverage Δ
lightning/src/chain/keysinterface.rs	95.04% <ø> (ø)
lightning/src/ln/channelmonitor.rs	95.70% <ø> (ø)
lightning/src/ln/onchaintx.rs	93.94% <ø> (ø)
lightning/src/util/enforcing_trait_impls.rs	100.00% <ø> (ø)
lightning/src/ln/chan_utils.rs	95.97% <75.00%> (-0.28%)	⬇️
lightning/src/ln/functional_tests.rs	97.00% <0.00%> (ø)

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[TheBlueMatt]

Mostly questions about use-cases.

[ariard]

Apart of unsafe feature in discussion, Concept ACK

[devrandom]

Removed message field visibility change - will be a separate PR.

Added more detailed commit messaging and removed the draft marker.

[TheBlueMatt]

I'm fine with this as-is, though I think we need to newtype TxCreationKeys because the APi definitely encourages "unsafe trusting" of the in-RL-derived keys when doing external signing.

I'd prefer a more descriptive name over unsafe, though. Maybe unsafe_revoked_tx_generation or something?

[devrandom]

I renamed unsafe to unsafe_revoked_tx_signing.

I think the PreCalculatedTxCreationKeys change should go in a separate PR, because the changes (especially involving LocalCommitmentTransaction) will dwarf the other things in this PR. The PreCalculatedTxCreationKeys changes ended up smaller that I predicted, please take a look at #664.

[TheBlueMatt]

Looks good except for new_missing_local_sig, which could use a bit of clean up if its going to be pub.