@bitflags is too permissive with input expressions

[Seelengrab]

Due to refactoring @bitflags to use the same backend as @bitfield, the following doesn't error anymore:

julia> @bitflags struct Foo
           a:2
           _
           b
       end

and produces a Foo with two fields, breaking the invariant that the non-padding fields of a @bitflags object are always a single bit wide.