Add many string features

Add many string features

This patch improves the string features of Normand.

Specifically, this patch:

• Adds support for the Latin-1 to Latin-10 string encodings:

      s:latin1"bonne journée!"

      62 6f 6e 6e 65 20 6a 6f  75 72 6e e9 65 21  ┆ bonne journ•e!

  The `s:` prefix is needed and also works with existing UTF encoding
  names:

      s:u32le "yo la gang 👋"

• Adds the variable string item.

  The syntax is one of:

      u16le{some_expr}

      {some_expr : s:u16le}

  Any `bool`, `int`, `float`, or `str` variable/label is accepted in the
  expression.

  For the suffix encoding form, the `s:` prefix is required.

• Makes it possible to use a string, possibly literal, as a macro
  expansion parameter:

      m:meow(42, "mix")

• Adds the `--var-str` CLI option to add a variable having a string
  value:

      normand '--var-str=my_var=salut la gang' [...]

See the updated `README.adoc` for more details.

Adding tests and updating existing ones.

Change-Id: I01426bb5ffa53097bb8c85937f54f1b4b498ec06
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/normand/+/11019
Tested-by: jenkins <jenkins@lttng.org>