Use explicit imports, not `using FASTX`

[jakobnissen]

Writing using FASTX (or any other package) is bad for two reasons:

• If FASTX in a new version exports a new name, it may conflict with an existing names in ReadDatastores, causing an error. This means that even adding new features to FASTX can break this package
• It makes it harder to find the definition the used functions

All of these should be replaced with using FASTX: Foo, Bar, baz, or, if FASTX internals is used, using FASTX: FASTX, Foo, Bar, baz, such that e.g. FASTX.qux can be accessed.