added rsplit. fix rsearch for case when both string and pattern are empty

`rsplit` behaves like `split` but in the reverse direction:
````
julia> rsplit("foo,bar,baz", ',', 2)
2-element String Array:
 "foo,bar"
 "baz"    

julia> split("foo,bar,baz", ',', 2)
2-element String Array:
 "foo"    
 "bar,baz"
````

before `rsearch` fix:
````
julia> search("", "")
1:0

julia> rsearch("", "")
ERROR: BoundsError()
 in _rsearch at string.jl:224
 in rsearch at string.jl:264
````

after `rsearch` fix:
````
julia> rsearch("", "")
1:0
````