Fix FBStringTest build failure and hash computation for wchar

Summary:
Getting some build errors when doing `buck test folly/...`. This seems to be fixing it.

  folly/test/FBStringTest.cpp:1290:23: error: unknown type name 'basic_fbstring'; did you mean 'basic_fstream'?
    using u16fbstring = basic_fbstring<char16_t>;
                        ^~~~~~~~~~~~~~
                        basic_fstream

Also the test `testHashChar16` was failing because only part of the string is used when computing hash.

Reviewed By: yfeldblum

Differential Revision: D3575858

fbshipit-source-id: cdd5bdb9653d50beaf0ec82b659d31354b345441

folly/FBString.h

@@ -2440,9 +2440,9 @@ _GLIBCXX_END_NAMESPACE_VERSION
 
 #define FOLLY_FBSTRING_HASH1(T)                                        \
   template <>                                                          \
-  struct hash< ::folly::basic_fbstring<T> > { \
+  struct hash<::folly::basic_fbstring<T>> {                            \
     size_t operator()(const ::folly::basic_fbstring<T>& s) const {     \
-      return ::folly::hash::fnv32_buf(s.data(), s.size()); \
+      return ::folly::hash::fnv32_buf(s.data(), s.size() * sizeof(T)); \
     }                                                                  \
   };
 

folly/test/FBStringTest.cpp

@@ -1287,7 +1287,7 @@ TEST(FBString, testHash) {
 
 #if FOLLY_HAVE_WCHAR_SUPPORT
 TEST(FBString, testHashChar16) {
-  using u16fbstring = basic_fbstring<char16_t>;
+  using u16fbstring = folly::basic_fbstring<char16_t>;
   u16fbstring a;
   u16fbstring b;
   a.push_back(0);