Make generated C code as `const` as possible

Make generated C code as `const` as possible

This patch changes `gen.py` and many C-generating templates to make
barectf generate C code with as many `const` variables as possible.

The Jinja 2 `ft_c_type` filter now accepts a parameter to make the
returned C type `const`.

All the `*_params_str` filters also do; the reason is that I don't want
the public header to show those useless `const` parameters (for
variables), but I want the same parameters to be `const` in the C source
file.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>