Use options bag parameters rather than MsgParams type (#204)

The signing and encryptuon functions all accepted a `MsgParams`
parameter that included just a single property (`data`, the data to be
signed). This pointless wrapper object around this single parameter
complicated the type signatures of these functions.

Instead, these functions now accept all parameters as one "options bag"
parameter, and the `MsgParams` type has been removed. This makes the
functions easier to use, as now it's not possible to enter the
parameters in the wrong order.

The related `SignedMsgParams` type has also been replaced in the same
manner, for the same reasons.