basic_string.h (front, back): Add.

2010-05-31  Paolo Carlini  <paolo.carlini@oracle.com>

	* include/bits/basic_string.h (front, back): Add.
	* include/debug/string: Adjust.
	* include/ext/vstring.h (front, back): Tweak the implementation
	to follow more closely the letter of the specs.
	* testsuite/21_strings/basic_string/element_access/char/
	front_back.cc: New.
	* testsuite/21_strings/basic_string/element_access/wchar_t/
	front_back.cc: Likewise.
	* config/abi/pre/gnu.ver: Export new symbols.
	* testsuite/util/testsuite_abi.cc: Adjust.
	* configure.ac: Bump minor version to 6:15:0.
	* configure: Regenerate.

From-SVN: r160071