Add some new benchmark tests to libstd/vec

More new benchmark tests. Two of them are benchmarking `starts_with` and `ends_with`.
Let me know if I am missing something.
Thanks in advance.

```
$ ./std --bench starts_with

running 3 tests

test str::tests::test_starts_with ... ignored
test vec::tests::test_starts_with ... ignored
test vec::bench::starts_with_incremental_length                 ... bench:    325717 ns/iter (+/- 4632)

test result: ok. 0 passed; 0 failed; 2 ignored; 1 measured

$ ./std --bench ends_with

running 3 tests

test str::tests::test_starts_with ... ignored
test vec::tests::test_starts_with ... ignored
test vec::bench::starts_with_incremental_length                 ... bench:    325717 ns/iter (+/- 4632)

test result: ok. 0 passed; 0 failed; 2 ignored; 1 measured

running 5 tests
test path::posix::tests::test_ends_with_path ... ignored
test path::windows::tests::test_ends_with_path ... ignored
test str::tests::test_ends_with ... ignored
test vec::tests::test_ends_with ... ignored
test vec::bench::ends_with_decremental_length                   ... bench:    329148 ns/iter (+/- 2019)

test result: ok. 0 passed; 0 failed; 4 ignored; 1 measured

$ ./std --bench push

running 17 tests
test io::extensions::test::push_bytes ... ignored
test io::extensions::test::push_bytes_eof ... ignored
test io::extensions::test::push_bytes_error ... ignored
test io::extensions::test::push_bytes_fail_reset_len ... ignored
test io::extensions::test::push_bytes_partial ... ignored
test path::posix::tests::test_push ... ignored
test path::posix::tests::test_push_many ... ignored
test path::posix::tests::test_push_path ... ignored
test path::windows::tests::test_push ... ignored
test path::windows::tests::test_push_many ... ignored
test path::windows::tests::test_push_path ... ignored
test str::tests::test_push_byte ... ignored
test str::tests::test_push_char ... ignored
test str::tests::test_push_str ... ignored
test vec::tests::test_push ... ignored
test str::bench::bench_push_str                                 ... bench:       685 ns/iter (+/- 3)
test vec::bench::push                                           ... bench:      5828 ns/iter (+/- 21)

test result: ok. 0 passed; 0 failed; 15 ignored; 2 measured
```