Let map-related functions be templated over key types

Let map-related functions be templated over key types

Summary:
[Folly] Let map-related functions be templated over key types.

This supports the case where map members like `find` may be overloaded or templated on types other than the map's `key_type const&`.

In C++14, standard library map types gain template overloads for `find` and for other members, permitting lookups without constructing `key_type` instances. This is useful primarily when `key_type` is expensive but another object type comparable with `key_type` is inexpensive. As a common example, a `key_type` of `std::string` is expensive in the general case because it allocates storage on the heap but `std::string_view` (C++17), when constructed with non-allocated `char const*, std::size_t`, would be inexpensive.

Reviewed By: terrelln

Differential Revision: D5145879

fbshipit-source-id: 979b75bfe55661aab11d5302da1bcd7830abd5af