Fix FBStringTest build failure and hash computation for wchar

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