Core: Remove using namespace statements from the Jit and Interpreter headers

[lioncash]

Moves them to the cpp files so we don't have these sort of things drift via header.

[Tilka]

lgtm

[BhaaLseN]

Should probably have been in the code style PR, but whats our general rule for using using namespace NS; vs. using NS::Member;?
Removing them in once place but adding them elsewhere doesn't really make much sense at first sight. And its pretty unlikely that we have duplicate defines somewhere (that may conflict with global ones).

[Tilka]

@BhaaLseN

Removing them in once place but adding them elsewhere doesn't really make much sense at first sight.

It's just that you don't want them in headers where they can end up all over the place.

[BhaaLseN]

Duh, point taken. I tend to get focused on the actual changes so I don't really mind what kind of files they're in.