Rename signTypedData-related types.

The type `EIP712TypedData` has been renamed to `TypedDataV1Field` so
that it does not get confused for a valid `signTypedData` message or
field. This type is only used for `V1` of `signTypedData`, which was
based on an early draft version of EIP-712 and doesn't reflect the
state of that EIP today at all.

The type `Version` has been renamed to `SignTypedDataVersion`. This
ensures it doesn't get confused for an encryption version, for example.
It's directly exported from this package, so it might not be clear at
first that it's related to `signTypedData`.