int hashCode(K key),
bool isValidKey(potentialKey)}
Creates an insertion-ordered hash-table based Map.
If equals is provided, it is used to compare the keys in the table with
new keys. If equals is omitted, the key's own Object.operator== is used
instead.
Similar, if hashCode is provided, it is used to produce a hash value
for keys in order to place them in the hash table. If it is omitted, the
key's own Object.hashCode is used.
If using methods like operator[], remove and containsKey together
with a custom equality and hashcode, an extra isValidKey function
can be supplied. This function is called before calling equals or
hashCode with an argument that may not be a K instance, and if the
call returns false, the key is assumed to not be in the set.
The isValidKey function defaults to just testing if the object is a
K instance.
The used equals and hashCode method should always be consistent,
so that if equals(a, b) then hashCode(a) == hashCode(b). The hash
of an object, or what it compares equal to, should not change while the
object is in the table. If it does change, the result is unpredictable.
If you supply one of equals and hashCode,
you should generally also to supply the other.
An example would be using identical and identityHashCode,
which is equivalent to using the shorthand LinkedHashMap.identity).