bug fix with string literals in a code procedure. Refs: #43.
Although currently not used in BB itself, string literals may
appear in a code procedure. The change for the representation
of code procedures must also be applied for string literals.
In addition:
1. It is now possible to have any number of string literals
as specified in the docu.
2. Usage of real constants inside a code procedure is marked as an error now.

Signed-off-by: Josef Templ <josef.templ@gmail.com>