Allow safe concurrent access to (mock.Mock).Calls

[brackendawson]

Like #965, I bumped into a need to safely access (mock.Mock).Calls today, but I wanted the calls rather than the length.

I think either a method to copy the Calls and return them:

// GetCalls returns a copy of the current Calls array.
func (m *Mock) GetCalls() []*Call

Or a high order method to safely access the Calls slice:

func (m *Mock) CallsFn(f ([]*Call)) {
    m.mutex.Lock()
    defer m.mutex.Unlock()
    f(m.Calls)
}

[brackendawson]

Related to #944