This also unifies `get_default_value` helper across all the target
languages.
Before this, there were two different versions of it for python
and typescript while c++ side didn't have one at all.
Note that other code-generating TTG-backends (namely python and
typescript) emitting ones already support this. This is just a matter
of extending similar logic to the c++ backend.
The `namespace` property is strictly used just by the cpp generator.
Move the code so the `--namespace` option is exclusive to it and removed
for all the other generators.
Merge all the command-line scripts for generating tuple-tree
implementations into a single `tuple-tree-generate.py` script. Move all
the other files a directory down.
Giacomo Vercesi