Rename crates to use underscores

[cycraig]

Description of change

Renames all projects to use underscores for their package and directory names.

• identity -> identity_iota
• identity-account -> identity_account
• identity-account-storage -> identity_account_storage
• identity-core -> identity_core
• identity-credential -> identity_credential
• identity-did -> identity_did
• identity-iota -> identity_iota_client
• identity-iota-core -> identity_iota_core

NOTE: this excludes the identity-diff and identity-diff-derive packages since those names are already fixed.

Notably, the root package is now named identity_iota. The root module for identity_iota_client (formerly identity_iota) changed from ::iota to ::client.

Updated documentation wherever possible but may have missed something.

Links to any relevant issues

Part of #869.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Tests etc. seem to pass locally.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[abdulmth]

Looks good, minor questions.

[eike-hass]

Ran through builds, test and examples in the core and bindings. Everything seems ok.
I reviewed the workflows for any references to the old structure, everything looks good here as well.
Great job! Thank you 🙏

[eike-hass]

Oh, one minor thing after running the stronghold-nodejs build locally the Cargo.lock of that project changed quite a bid. Does that change belong in this PR?

Edit: Had to npm link to the Wasm bindings to get the build working, but that should not affect the cargo dependencies.