Improve is_palindrome performance significantly (C++03 compat)

[jeking3]

Refactored is_palindrome to use std::equal yielding a 58% performance gain in limited testing (x86_64, gcc 12.2). This has fewer branches and leverages the optimization already done on the std::equal implementation.

Before: 6.953285 secs

jking@pulsar:~/boost/libs/algorithm/performance$ ../../../bin.v2/libs/algorithm/performance/perf_is_palindrome.test/gcc-12/release/x86_64/link-static/perf_is_palindrome
Total runs: 12000000
Total palindromes found: 6000000
Total elapsed time: 6.95016 seconds
Average time per is_palindrome: 0.57918 microseconds
jking@pulsar:~/boost/libs/algorithm/performance$ ../../../bin.v2/libs/algorithm/performance/perf_is_palindrome.test/gcc-12/release/x86_64/link-static/perf_is_palindrome
Total runs: 12000000
Total palindromes found: 6000000
Total elapsed time: 6.95641 seconds
Average time per is_palindrome: 0.579701 microseconds

After: 2.915925 secs

jking@pulsar:~/boost/libs/algorithm/performance$ ../../../bin.v2/libs/algorithm/performance/perf_is_palindrome.test/gcc-12/release/x86_64/link-static/perf_is_palindrome
Total runs: 12000000
Total palindromes found: 6000000
Total elapsed time: 2.92422 seconds
Average time per is_palindrome: 0.243685 microseconds
jking@pulsar:~/boost/libs/algorithm/performance$ ../../../bin.v2/libs/algorithm/performance/perf_is_palindrome.test/gcc-12/release/x86_64/link-static/perf_is_palindrome
Total runs: 12000000
Total palindromes found: 6000000
Total elapsed time: 2.90763 seconds
Average time per is_palindrome: 0.242302 microseconds

Yielding a 58.06% performance gain.