refactor: rename inner field names

[reez]

Description

Rename non-public inner struct fields as "inner"

Notes to the reviewers

Linked Issue and description

Describe the enhancement
In a few places we need to "wrap" a bdk or rust-bitcoin struct in a bkd-ffi struct and make the original type a non-public "inner" field. We've used different names for these inner fields but should standardize on the name "inner" for consistency and readability. Examples of stucts that need to be changed:
Transaction.internal
Address.address
Script.script
Blockchain.blockchain_mutex
Mnemonic.internal
DerivationPath.derivation_path_mutex
DescriptorPublicKey.descriptor_public_key_mutex
DescriptorSecretKey.descriptor_secret_key_mutex
Input._inner
PartiallySignedTransaction.internal
Wallet.wallet_mutex

Use case
Users won't see this change.

Additional context
The name "inner" is common in other libraries like rust-bitcoin.

I looked thru the list of items referenced in the Issue that needed to be changed:

• Transaction.internal
• Address.address
• Script.script
• Blockchain.blockchain_mutex
• Mnemonic.internal
• DerivationPath.derivation_path_mutex
• DescriptorPublicKey.descriptor_public_key_mutex
• DescriptorSecretKey.descriptor_secret_key_mutex
• Input._inner
• PartiallySignedTransaction.internal
• Wallet.wallet_mutex

The only item from that list I didn't change was Input._inner because I couldn't find Input. I tried to do some digging to see if Input was changed to something else but I didn't quite find it.

I did not make any changes to the bdk.udl file

Let me know if there is anything that should/shouldn't be changed that I missed.

Also if this gets approved I'm totally happy to wait on it being merged if #392 needs to be merged first or anything.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

I couldn't find where that Input._inner was either, so don't worry about it.

[reez]

Thanks for the helpful review and guiding on this @notmandatory , I updated the code to reflect the changes requested (CI + tests pass).

[thunderbiscuit]

Tested ACK 7717ebb. I like the idea of identifying the mutexes by naming them inner_mutex.

Good to go IMO. I'll rebase #392.