token api: reduce the number of allocated structs

[adecaro]

For example, every time we retrieve the public parameters using the Token API, we execute the following code:

func (c *PublicParametersManager) PublicParameters() *PublicParameters {
	pp := c.ppm.PublicParameters()
	if pp == nil {
		return nil
	}
	return &PublicParameters{PublicParameters: pp, ppm: c.ppm}
}

that instantiates a new struct. This struct can be instantiated once at the beginning and reused over and over again.