Replace std::size_t return types in F14 to size_type

Summary:
The heterogenous count() method currently returns size_type, unlike
similar methods (normal count(), F14Map.count() methods).  This
standardizes it.

Reviewed By: nbronson

Differential Revision: D14426807

fbshipit-source-id: 140be39fcda99c9f82ced3962ddcbdd1d5f53ba6

folly/container/F14Map.h

@@ -747,12 +747,12 @@ class F14BasicMap {
     return try_emplace(std::forward<K>(key)).first->second;
   }
 
-  FOLLY_ALWAYS_INLINE std::size_t count(key_type const& key) const {
+  FOLLY_ALWAYS_INLINE size_type count(key_type const& key) const {
     return table_.find(key).atEnd() ? 0 : 1;
   }
 
   template <typename K>
-  FOLLY_ALWAYS_INLINE EnableHeterogeneousFind<K, std::size_t> count(
+  FOLLY_ALWAYS_INLINE EnableHeterogeneousFind<K, size_type> count(
       K const& key) const {
     return table_.find(key).atEnd() ? 0 : 1;
   }

folly/container/F14Set.h

@@ -529,7 +529,7 @@ class F14BasicSet {
 
   //// PUBLIC - Lookup
 
-  FOLLY_ALWAYS_INLINE std::size_t count(key_type const& key) const {
+  FOLLY_ALWAYS_INLINE size_type count(key_type const& key) const {
     return table_.find(key).atEnd() ? 0 : 1;
   }