Implement CRC-32 elements & verification

[FreezyLemon]

I found what made the tests from #106 fail: CRC elements aren't handled in the EBML header.

Changes:

1. Add parse_binary_exact to get an exact amount of bytes in an array
2. Add combinator-style functions to get the checksum from the input (fn crc) and to verify the data with the checksum (fn checksum)
   (2.5) Add the crc crate to calculate the checksum(s) as per the specification.
3. Use these functions in ebml_header and sub_element

The complete error handling for CRC-32 is a bit more involved than just returning an error, but I'd say it's good enough for now.

[Luni-4]

It's fine for me! Thanks a lot!