Removes the Options around the function pointers

None of the JNI 1.1 function pointers were optional, and neither
are any of the function pointers in later versions of the spec.

Having the `Option` implies that null pointer checks are required
but, more notably, they also suggest that null pointer checks could
be used for JNI >= 1.2 pointers, which could be a dangerous mistake
since these are effectively extending beyond the table of pointers
that was defined for JNI 1.1 and should be assumed to be invalid
pointers that mustn't be touched.

It's also notable that we sometimes have to call `GetVersion` to
determine the full set of pointers that are valid.

Recently the use of `Option` also raised some questions about our
ability to, infallibly, handle Rust panics when we want to map
a panic to a Java exception via JNI:

jni-rs/jni-rs#432 (comment)