Allow subscripting raw pointers
    
    This will be useful for dealing with vectors; regardless of our final solution
    for the Index trait.
    
    2016-07-06  Manish Goregaokar  <manish@mozilla.com>
    
    gdb/ChangeLog:
        * rust-lang.c (rust_subscript): Allow subscripting pointers
    
    gdb/testsuite/ChangeLog:
        * simple.rs: Add test for raw pointer subscripting
        * simple.exp: Add test expectations