Add SortedStringValues and SortedUniqueStringValues

Add SortedStringValues and SortedUniqueStringValues that return the
values of a string-valued map.  Also make the SortedStringKeys
function use a similar implementation with MapRange, which avoids
iterating over the keys twice, once in MapKeys to build the list
of key reflect.Values and once over the reflect.Values to convert
them to strings.

Test: util_test.go
Change-Id: I4fc990a5036421e8926094ee158fafe606d0f10b