Changes relative to the proposal:

• Re-ordered the basic_string literals so they are listed in the
  order char, char16_t, char32_t and wchar_t. This matches the other
  functions in Clause 21.
• Changed const char *str to const char* str for consistency with
  asterisk placement in basic_string itself.
• Changed operator"" to operator "" for consistency with core wording.
• Added whitespace to duration literal declarations so they are aligned
• Changed section title to "Suffixes for duration literals" instead of
  "Suffix for duration literals"
• Use string, u16string etc. typedefs for basic_string