syntax: add noinline/inline support overrides for structs
An example:
```julia
julia> @noinline struct NoinlineSingleton end
julia> @inline struct NormalSingleton end
julia> @inline struct Impossible; x::Impossible; end
ERROR: Cannot apply @inline to this struct which is self-referential
Stacktrace:
[1] top-level scope
@ REPL[5]:1
julia> sizeof(Tuple{NoinlineSingleton})
8
julia> sizeof(Tuple{NormalSingleton})
0
```