Add convenience func and args

[thomaspinckney3]

I found these helpful in a recent project. Not sure if there is a better way to do this. This is my first pull request so hopefully getting it right!

types.py: add get_element_offset to Type values.py: add packed arg to literal_struct context: add packed arg to get_identified_type tests: add test cases

[gmarkall]

Thanks for the PR! I need to look over the code more closely, but I think the idea of these is good in general.

[gmarkall]

I've added this to the next milestone since it seems like it should be pretty easy to get in before the next release.

[gmarkall]

Many thanks for your contribution, @thomaspinckney3! This will be included in the next release of llvmlite (v0.44.0).