lib.parse: use TryInto trait to replace peek to make parse not need std

[Evian-Zhang]

This PR is to avoid std for the parse method of Object.

The reason that parse need std is peek needs it, and peek's task is:

1. take the first 16 bytes of the total bytes;
2. pass the taken array of 16 bytes to peek_bytes method.

And there is a no-std way to take the first 16 bytes of the total bytes:

fn take_hint_bytes(bytes: &[u8]) -> Option<&[u8; 16]> {
    use core::convert::TryInto;
    bytes.get(0..16)
        .and_then(|hint_bytes_slice| {
            hint_bytes_slice.try_into().ok()
        })
}

The core magic is get (which takes the first 16 elements of the given slice and returns a slice) and the TryInto trait (which converts the slice to array type, see more in How to get a slice as an array in Rust?).

For compatibility reason, I don't delete peek method although it is dead code.

[m4b]

LGTM very much appreciated!!

[m4b]

Wow this is great, when did this get merged into rust ? (Do you know offhand which rustc this requires ?)

[Evian-Zhang]

TryInto trait was stabilized in Rust 1.34

[m4b]

Uh this is awesome, this todo has been here a very long time, and I really appreciate you fixing it! Great work :)

[m4b]

Thanks so much @Evian-Zhang ! :D