Use only naked `{` and `}` for variable assignment

Use only naked `{` and `}` for variable assignment

This patch:

* Makes a fixed-length number, an LEB128, or a string item use
  `[` and `]` instead of `{` and `}`.

* Changes `{be}` and `{le}` to `!be` and `!le`.

* Removes the `[` and `]` symbols from the insignificant symbol set
  (I knew this day would come).

This means anything between `{` and `}` is now a valid Python 3
expression. A side effect of this patch is to also simplify the parser
because when expecting an item, `{` already means a variable assignment.

`README.adoc` and tests are changed accordingly.

Change-Id: I071978b0eae9349b779be44843e5f8dd0a162461
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/normand/+/11036
Tested-by: jenkins <jenkins@lttng.org>