err: tweak names, inlining and docs

[davidhewitt]

This PR cleans up a bunch of naming on the PyErr type:

• make get_type, value and traceback as per Better names for PyErr::ptype, PyErr::pvalue and PyErr::ptraceback #1987
• replace all mentions of "instance" with "value" for consistency (into_instance -> into_value and from_instance -> from_value).
• improve docs on these methods a little
• add #[inline] in a couple of places where it looked like it made sense. According to bench_err this has made a slight improvement.

PyErr::into_instance (now PyErr::into_value) actually had a memory leak in it, which is now also fixed.

Closes #1987