Implement P0457R2 String Prefix and Suffix Checking.

Implement P0457R2 String Prefix and Suffix Checking.

2018-11-30  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Implement P0457R2 String Prefix and Suffix Checking.
	* include/bits/basic_string.h: Add starts_with, ends_with members.
	* include/std/string_view: Ditto.
	* testsuite/21_strings/basic_string/operations/starts_with/
	char/1.cc: New test.
	* testsuite/21_strings/basic_string/operations/starts_with/
	wchar_t/1.cc: New test.
	* testsuite/21_strings/basic_string/operations/ends_with/
	char/1.cc: New test.
	* testsuite/21_strings/basic_string/operations/ends_with/
	wchar_t/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/starts_with/
	char/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/starts_with/
	wchar_t/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/ends_with/
	char/1.cc: New test.
	* testsuite/21_strings/basic_string_view/operations/ends_with/
	wchar_t/1.cc: New test.

From-SVN: r266674