Change IotaDocument::verify_document from a static function to a method

[cycraig]

Description of change

Changes the static associated function IotaDocument::verify_document(signed: &IotaDocument, signer: &IotaDocument) to be a method on IotaDocument, as suggested in #674. This change is to prevent confusion on the order of parameters in the function signature, as confusing signed with signer could lead to hard-to-detect bugs.

New signature: IotaDocument::verify_document(&self, signed: &IotaDocument)

This change also applies to WasmDocument.

This PR also changes the default build target for the bindings/wasm project to wasm32-unknown-unknown to ease local development friction when running cargo commands following the fixes in PR #666 (thanks @olivereanderson!).

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

Existing tests and examples which use verifyDocument explicitly and implicitly during DID Document resolution 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

[PhilippGackstatter]

Looks good to me, thanks for the changes and the wasm-target fix!