Fasta: create and use Tags sub-module
Fasta: put `item_per_line` in the Tags
Fasta: add unit_to_random_char_seq_raw_item

The transform uses the tags to generate `char_seq raw_item` values.
app.random: add support for FASTA files

The way the command line specification is parsed is a bit hackish for
now; it is a transition between the currend ad-hoc mode and the future
tags DSL.
Tags,Fasta: put `Fasta.Tags.t` in `Tags.t`
Fasta: add missing documentation