Add `load_or_new` function that loads a DB if available, and if not, create DB

[sigaloid]

[seladb]

Thanks for your contribution @sigaloid ! Can you please fix the clippy errors before we merge your PR?

Also - it'd be great if you can add a test.

[sigaloid]

Hi, I think I've fixed the clippy error. Also, I'm unfamiliar with the test structure so a test that I write would likely not help very much or be poorly written :pensive:

[seladb]

@sigaloid I'm sorry for the late response. I think you can add a simple test here: https://github.com/seladb/pickledb-rs/blob/master/tests/pickledb_key_value_tests.rs

Also - the clippy errors were fixed here: https://github.com/seladb/pickledb-rs/pull/25 so you can update your branch and it should work now

[sigaloid]

Successfully added tests, looks like it's passing in my repo but needs a maintainer to run here.

[seladb]

@sigaloid please see a few clippy and fmt issues