introduce a character type class in typelib

[doudou]

The handling of C 'char' is currently very broken in typelib, as typelib represents it as a numerical type. Among the few consequences of that choice:

• the type has to be signed, which is platform-dependent. This does not need to be represented on the typelib side, as the representation of 'A' is the same at the binary level, regardless of whether, in C, 'A' is a positive or negative number
• when displaying a value, typelib does not know whether it should be displayed as a numerical type or a character (while it is unambiguous from the type's definition). This leads to the infamous problem that the command-line pocolog tools displays arrays of chars as arrays of values.