- change value_t::typename from CamelCase to snake_case.
- drop CamelCase typename supports.
The changes are introduced to make the interfaces uniform. For some
(historical) reasons, toml11 has both CamelCase names and snake_case
names for types. Additionally, since `float` is a keyword, snake_case
names uses `floating` to avoid collision and CamelCase name uses `Float`
because toml official calls it `Float`. This is too confusing.
Since it is a major upgrade, I think it is a big chance to make them
uniform.
