Workaround for stack overflows with has_reflect string

The default has_reflect string instance in Lean only work for strings up to few thousand characters. Anything larger than that will trigger a stack overflow because the string is represented as a very deeply nested expression: https://github.com/leanprover-community/lean/issues/144

This file adds a higher-priority instance for has_reflect string, which behaves exactly the same for small strings (up to 256 characters). Larger strings are carefully converted into a call to string.join.