It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.
Closes#1733.
Use explicitly defaulted destructors for base cases when possible,
eliminate aggregate initialization constructors where possible, and push
some more stuff to NSDMIs.
Make the vtables less absurdly huge (knocks 100KB off aegisub32.exe),
eliminate some copies of the values when constructing the options, and
use an enum class for the value type.
