Fix two bugs introduced in 79f89dd

- Closure._convert_libjit_to_ctypes missed two return statements when
  converting aggregate types.
- When creating ctypes wrappers for aggregate types, the `_fields_'
  attribute must be a list of 2-tuples specifying the name and type of
  the individual fields. In the last commit, however, we only assembled
  the types in a list and forgot to give unique field names.
- Testing for primitive types in Closure._convert_libjit_to_ctypes means
  we cannot convert things like jit.Type.SYS_CHAR since LibJIT does not
  consider SYS_* types primitive. We skip the primitivity test altogether now
  to avoid confusion.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Add a few tests for closure type conversions

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
README.markdown: fix small documentation hiccup

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>