=head2 method keyof

Defined as:

    method keyof

Usage:

    Hash.keyof

Returns the type constraint for the keys of the invocant. For
normal hashes the method returns the coercion type C<(Str(Any))>
while for L<object hashes|/type/Hash#Object_hashes_and_type_constraints>
the type used in the declaration of the C<Hash> is returned.

    my %h1 = 'apples' => 3, 'oranges' => 7;  # (no key type specified)
    say %h1.keyof;                           # (Any)

    my %h2{Str} = 'oranges' => 7;            # (keys must be of type C<Str>)
    say %h2.keyof;                           # (Str)