Make SetCurve a member of the Party, not global to the lib

[notatestuser]

Right now the curve that the library uses is set through a call to tss.SetCurve(...) and a reference to the given curve is stored globally, applying to all active Party instances.

It would be better for the curve to be a member of the BaseParty, set through a call to a new SetCurve function on an individual Party.

[0xmountaintop]

It will also be better if "LocalPartySaveData" can be abstract a bit more.

Currently common.SignatureData and tss.Party are already abstract, so different type signing party can be started according to the desired curve, similarly, SignatureData can be serialized according the curve.

it will be nicer if "LocalPartySaveData" is also abstract, so user don't need to worry about worry about the outCh type.