Implement generic methods for commonly used functions and export them

[harryscholes]

Types of changes

This PR implements the following changes: (Please tick any or all of the following that are applicable)

• [x] :sparkles: New feature (A non-breaking change which adds functionality).
• [ ] :bug: Bug fix (A non-breaking change, which fixes an issue).
• [ ] :boom: Breaking change (fix or feature that would cause existing functionality to change).

:clipboard: Additional detail

• If you have implemented new features or behaviour
  • Provide a description of the addition in as many details as possible.
• Implement generic methods for commonly used functions
• Add methods to them in FASTA and FASTQ

• Export them from FASTX so that they are available after using FASTX 😄

  • Provide justification of the addition. I often find myself having to write stuff like:

FASTA.identifier(record)
FASTA.description(record
FASTA.sequence(record)

which is a bit annoying. Not massively annoying, but a bit annoying.

Also, due to lovely stuff like multiple dispatch, it feels more Julian to use generic methods that are not hidden in a namespace. This will allow cleaner code and more consistent APIs.

The same could be done for lots of other functions that are common to the FASTA and FASTQ modules. I just started with these as a proof of concept. Would be keen to hear people's thoughts on this!

• Provide a runnable example of use of your addition. This lets reviewers and others try out the feature before it is merged or makes it's way to release.

  
  using FASTX

record = FASTA.Record("abc", "def", "ACGT") identifier(record) description(record) sequence(record)

record = FASTQ.Record("abc", "def", "ACGT", collect("JJJJ")) identifier(record) description(record) sequence(record) quality(record)

- If you have changed current behaviour...
  - **Describe the behaviour prior to you changes**
These methods had to be accessed from the `FASTA` and `FASTQ` namespaces.

  - **Describe the behaviour after your changes** and justify why you have made the changes,
    Please describe any breakages you anticipate as a result of these changes.
The methods are now exported by `FASTX`. No breaking changes.

  - **Does your change alter APIs or existing exposed methods/types?**
    If so, this may cause dependency issues and breakages, so the maintainer
    will need to consider this when versioning the next release.
Sort of, but it won't break anything.

## :ballot_box_with_check: Checklist

- [x] :art: The changes implemented is consistent with the [julia style guide](https://docs.julialang.org/en/stable/manual/style-guide/).
- [ ] :blue_book: I have updated and added relevant docstrings, in a manner consistent with the [documentation styleguide](https://docs.julialang.org/en/stable/manual/documentation/).
- [ ] :blue_book: I have added or updated relevant user and developer manuals/documentation in `docs/src/`.
- [ ] :ok: There are unit tests that cover the code changes I have made.
- [ ] :ok: The unit tests cover my code changes AND they pass.
- [ ] :pencil: I have added an entry to the `[UNRELEASED]` section of the manually curated `CHANGELOG.md` file for this repository.
- [x] :ok: All changes should be compatible with the latest stable version of Julia.
- [ ] :thought_balloon: I have commented liberally for any complex pieces of internal code.

[harryscholes]

I just removed the generic method for quality because only FASTQ defines it.

[codecov[bot]]

Codecov Report

Merging #11 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #11   +/-   ##
=======================================
  Coverage   88.76%   88.76%           
=======================================
  Files          13       13           
  Lines         445      445           
=======================================
  Hits          395      395           
  Misses         50       50

Impacted Files	Coverage Δ
src/fasta/fasta.jl	100% <ø> (ø)	:arrow_up:
src/fastq/fastq.jl	100% <ø> (ø)	:arrow_up:
src/FASTX.jl	100% <ø> (ø)	:arrow_up:

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