Expose libolm migration code in wasm bindings

[richvdh]

The FFI bindings have a bunch of code to take libolm-pickled objects and store them in a matrix-rust-sdk CryptoStore.

To support migration in Element-Web R, I think we need to push as much of this as we can down to matrix-sdk-crypto, and then expose it in the wasm bindings.

[bnjbvr]

Can you expand a bit? Have the libolm objects been created by the Rust SDK in the first place?

[richvdh]

Can you expand a bit? Have the libolm objects been created by the Rust SDK in the first place?

No, they are created by the application.

I don't really have any more detail at the moment. All I know is that there is a load of code in the FFI bindings, none of which looks very specific to FFI, and which we need to replicate in the wasm bindings. My hope is that we can save a lot of copying-and-pasting by moving this code to somewhere common, like matrix-sdk-crypto.

I plan to look at it later this week.