very slow jni code callbacks

[Tom Tromey]

Bryce> I think the underlying issue is that jMethodID is typedef'ed to
Bryce> libgcj's internal _Jv_Method. It really needs to use something more
Bryce> like the interpreter's _Jv_ResolvedMethod or _Jv_JNIMethod so that the
Bryce> arg types and return type (and, in fact, the ffi_cif itself) can be
Bryce> resolved once and reused.

I think there is some reason we use _Jv_Method, and not something else,
for the jMethodID typedef.  I don't remember what it was.

Tom