Clarify the `text` format.  Currently it serves two purposes:

- to present the user with a readable definition of the valueset.
  This is a failure: `lal_char(abc), b` is unpleasant to read.
  We would prefer `{abc} → 𝔹`.

- to build the snames.  This is a failure too, as we had to introduce
  a specific format (format::generators) to print generators, instead
  of `text`.

So rename `generators` as `sname`, and use the latter name instead of
`text` to build signatures.  Let's make `text` produce a human
readable result, using UTF-8.

* vcsn/misc/format.hh, lib/vcsn/misc/format.cc (generators): Rename as...
(sname): this.

* vcsn/alphabets/char.hh, vcsn/alphabets/setalpha.hh,
* vcsn/alphabets/string.hh, vcsn/core/rat/expansionset.hh,
* vcsn/core/rat/expressionset.hxx, vcsn/ctx/context.hh,
* vcsn/labelset/letterset.hh, vcsn/labelset/nullableset.hh,
* vcsn/labelset/oneset.hh, vcsn/labelset/tupleset.hh,
* vcsn/labelset/wordset.hh, vcsn/weightset/b.hh,
* vcsn/weightset/log.hh, vcsn/weightset/nmin.hh,
* vcsn/weightset/polynomialset.hh, vcsn/weightset/q.hh,
* vcsn/weightset/qmp.hh, vcsn/weightset/r.hh,
* vcsn/weightset/rmin.hh, vcsn/weightset/z.hh,
* vcsn/weightset/zmin.hh
(print_set): Be consistent in the support of `latex`, `sname`
and `text`.

* vcsn/algos/dot.hh: Adjust: ask for `sname`.
* tests/python/context.py: Likewise.