Avoid converting the return value to `JsValue` in `generate_kfrags()`

[fjarri]

A vector or a box of custom structs cannot currently be returned from a function (see wasm-bindgen/wasm-bindgen#111). So generate_kfrags() has to convert KeyFrags to JsValues, which will make testing harder. Need to fix it when the blocker is fixed.

[fjarri]

Now using a custom type VerifiedKeyFragArray declared as TS VerifiedKeyFrag[] and a manual conversion, to generate a correct TS signature. The internals can be updated when wasm-bindgen fixes the underlying issue.