very slow jni code callbacks

[Tom Tromey]

Andrew> This calls _Jv_GetTypesFromSignature, which in turn iterates though
Andrew> all the class loaders looking up the types of all the arguments.
Andrew> But as far as I can see this is a pointless waste of time: the only
Andrew> use of the type of each argument is to determine whether it's a
Andrew> primitive type or not, and we could certainly have done that when
Andrew> we created the jmethodID.

Yeah, this could use some improvement.
Even a variant of _Jv_GetTypesFromSignature that did less work would
help.

Tom