_CCodeGenerator.generate_header(): use Jinja 2 templates

_CCodeGenerator.generate_header(): use Jinja 2 templates

This patch makes _CCodeGenerator.generate_header() generate the main
barectf header file using Jinja 2 templates.

The master template, `barectf.h.j2`, includes:

`c-common.j2`:
    Common macros for C templates.

`c-ctx-init-func-proto.j2`:
    Context initialization function prototype.

`c-open-func-proto.j2`:
    Packet opening function prototype.

`c-close-func-proto.j2`:
    Packet closing function prototype.

`c-trace-func-proto.j2`:
    Tracing function prototype.

This patch removes a lot of code from `gen.py` and `templates.py` which
now has its equivalent in Jinja 2 templates.

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