Use TranscodingStreams for FASTA

:ballot_box_with_check: Checklist

[x] :art: The changes implemented is consistent with the julia style guide.
[x] :blue_book: I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
[x] :blue_book: I have added or updated relevant user and developer manuals/documentation in docs/src/.
[x] :ok: There are unit tests that cover the code changes I have made.
[x] :ok: The unit tests cover my code changes AND they pass.
[x] :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.
[x] :thought_balloon: I have commented liberally for any complex pieces of internal code.