simplify declaractions

[John-Colvin]

Don't want to allocate runtime space in TLS (or anywhere else) for IsVirtual, so change to enum

static isn't doing anything on CallParams

[jll63]

IsVirtual is used only a compile time. But indeed I saw in the std lib code that they use enum for this sort of things. static: dunni remember why, maybe I initially nested this class into another?? Regardless, thanks for the patch.

[PetarKirov]

@jll63 as a rule of thumb, if something is only needed at compile-time, always use enum. If the result would be used more than once at runtime, use static immutable, which in addition to what enum gives you (forcing compile-time evaluation) will store the literal as a global variable, so that multiple uses at runtime would not cause re-evaluation. See also: http://dlang.org/blog/2017/06/05/compile-time-sort-in-d/