Remove unchecked compile warnings

Removes compile warnings due to unchecked type conversion.

```
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentMap.java:28: warning: [unchecked] unchecked conversion
    [javac]       EMPTY = ClojurePersistentHashMap.create(c);
    [javac]                                               ^
    [javac]   required: Class<? extends Map>
    [javac]   found:    Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentMap.java:28: warning: [unchecked] unchecked method invocation: method create in class ClojurePersistentHashMap is applied to given types
    [javac]       EMPTY = ClojurePersistentHashMap.create(c);
    [javac]                                              ^
    [javac]   required: Class<? extends Map>
    [javac]   found: Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentSet.java:29: warning: [unchecked] unchecked conversion
    [javac]       EMPTY = ClojurePersistentHashSet.create(c);
    [javac]                                               ^
    [javac]   required: Class<? extends Set>
    [javac]   found:    Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentSet.java:29: warning: [unchecked] unchecked method invocation: method create in class ClojurePersistentHashSet is applied to given types
    [javac]       EMPTY = ClojurePersistentHashSet.create(c);
    [javac]                                              ^
    [javac]   required: Class<? extends Set>
    [javac]   found: Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/ClojurePersistentHashMap.java:64: warning: [unchecked] unchecked cast
    [javac]     return (V) map.get(key);
    [javac]                       ^
    [javac]   required: V
    [javac]   found:    Object
    [javac]   where V is a type-variable:
    [javac]     V extends Object declared in class ClojurePersistentHashMap
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/ClojurePersistentHashMap.java:69: warning: [unchecked] unchecked conversion
    [javac]     return map.keySet();
    [javac]                      ^
    [javac]   required: Set<K>
    [javac]   found:    Set
    [javac]   where K is a type-variable:
    [javac]     K extends Object declared in class ClojurePersistentHashMap
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/ClojurePersistentHashMap.java:79: warning: [unchecked] unchecked conversion
    [javac]     return map.entrySet();
    [javac]                        ^
    [javac]   required: Set<Entry<K,V>>
    [javac]   found:    Set
    [javac]   where K,V are type-variables:
    [javac]     K extends Object declared in class ClojurePersistentHashMap
    [javac]     V extends Object declared in class ClojurePersistentHashMap
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/ClojurePersistentHashSet.java:79: warning: [unchecked] unchecked conversion
    [javac]     return set.iterator();
    [javac]                        ^
    [javac]   required: Iterator<K>
    [javac]   found:    Iterator
    [javac]   where K is a type-variable:
    [javac]     K extends Object declared in class ClojurePersistentHashSet
    [javac] 8 warnings
```