Type hint toArray to fix JDK11 compatibility

[ameidatou]

On JDK11, toArray needs to be hinted
Syntax error (IllegalArgumentException) compiling deftype* at (hier_set/core.clj:24:1). Must hint overloaded method: toArray
should be similar to the fix in clj-commons/ordered

[bbss]

Fixed this:
#2

[jsavyasachi]

Is it possible to get this merged in?