After this commit, every non-key field is treated as if it was optional
while every key field (plus every auto-generated `Kind` field) - as if
it was required.
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.
Ivan Krysak