fix: ExpTaylor race condition

[Streppel]

Related: #368

Adds a RWMutex around the shared factorials slice.

[8xmx8]

Each call is computed independently; while it may be slightly slower, it is safer and clearer.

func (d Decimal) ExpTaylor(precision int32) (Decimal, error) {
	var factorials = []Decimal{New(1, 0)}
	// Note(mwoss): Implementation can be optimized by exclusively using big.Int API only
	if d.IsZero() {
		return Decimal{oneInt, 0}.Round(precision), nil
	}
···