Write a function, sum_list, that takes in the head of a linked list containing numbers as an argument. The function should return the total sum of all values in the linked list.
a = Node(2)
b = Node(8)
c = Node(3)
d = Node(-1)
e = Node(7)
a.next = b
b.next = c
c.next = d
d.next = e
# 2 -> 8 -> 3 -> -1 -> 7
sum_list(a) # 19x = Node(38)
y = Node(4)
x.next = y
# 38 -> 4
sum_list(x) # 42z = Node(100)
# 100
sum_list(z) # 100sum_list(None) # 0