Add Phred decoding functionality

[apcamargo]

This PR adds Phred decoding functionality to needletail. The following changes are introduced:

• quality module:
  • Adds a new quality module.
  • Adds the PhredEncoding enum, representing the type of encoding (Phred+33 or Phred+64).
  • Includes the decode_phred function, which takes Phred-encoded quality data (&[u8]) and a PhredEncoding instance, returning the decoded quality scores.
• errors module:
  • Adds the PhredOffsetError struct, representing a Phred-decoding error where the offset (33 or 64, depending on the encoding) exceeds the encoded quality value.
• SequenceRecord:
  • Adds a decode_phred method that returns a Option containing the decoded quality scores of FASTQ records. Decoded data is returned as a Cow<'a, [u8]>, which may be coerced to Vec<u8> or &[u8].

Tests are included for both to the quality::decode_phred function and record::SequenceRecord::decode_phred.

The motivation for this PR is to use the added functions in the following PR: #98

[audy]

Thanks!

[apcamargo]

Thanks, @audy! How are you planning to deal with this? Should I prepare the PR to add Phred decoding to the Python module or wait for you to merge this one?

[audy]

@apcamargo this one is good to merge. I'll merge it now and then you can update #92 if you like.