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.
Debugging is a lot harder when you cannot just diff outputs of
the generator.
This commit ensures types are always emitted in the same order,
no matter the input schema ordering or the target language.
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