hashtable_update(ht, key, fn, default)
Apply the function f to the value associated with key in the hash table ht,
or to default if no value is associated with key. If fn returns, key is
associated with the value returned by fn, replacing the old association.
Key should be of an appropriate type for the hash and equivalence functions used by the hash table.
The function fn should accept one argument, should return one value, and should not modify the hash table.
Hashtable_update return void.
let ht = #{1:2, 3:4}
hashtable_update(ht, 1, ^(x) x * 100)
ht
// #{1: 200, 3: 4}
hashtable_update(ht, 5, ^(x) x * 100, 6)
ht
// #{5: 600, 1: 200, 3: 4}