Support taking an array of cfrags in decrypt_reencrypted() in WASM bindings

[fjarri]

Vec<MyType> is not supported as a parameter in wasm-bindgen (see rustwasm/wasm-bindgen#111), so we have to work around that by building an ephemeral struct:

let plaintext_bob = capsule
    .with_cfrag(cfrag0)
    .with_cfrag(cfrag1)
    .decrypt_reencrypted(bob_sk, alice_pk, ciphertext);

That won't work very well when the threshold is not known statically.

[fjarri]

For now we're using a custom type (VerifedCapsuleFragArray, declared as TS VerifiedCapsuleFrag[]) and a manual dynamic conversion to js_sys::Array and further to VerifedCapsuleFrag using wasm-bindgen-derive. When wasm-bindgen fixes the underlying issue, the internals can be updated too.