`std::variant` is the last archetypal container missing in TreeBuilder-v2. The
code for it isn't hugely complicated and relies on pack expansion.
This change introduces a new field to the container specification:
`scoped_extra`. This field allows you to write extra code that will be included
within the TypeHandler in CodeGen. This means it will not have collisions with
other containers, unlike the existing `extra` field. It's used here to write
the recursive `getSizeType` function for `std::variant`.
Tech debt is introduced here by comparing the container name to `std::variant`
in CodeGen to conditionally generate some code. We've worked hard to remove
references to containers in code and move them to `.toml` files. On balance,
this is worth having to include the example of `std::variant`. It should be
moved into a container spec field at some point, the design of which is still
to be determined.
Test plan:
- Activated the OIL `std::variant` tests.
- CI
