Compare the "main salsa" runtime with the "salsa 2022" runtime and identify differences

[nikomatsakis]

The function ingredient in salsa 2022 was based off of the code from "old salsa", but there were also some improvements made after the fork. This issue exists to track the process of doing a detailed diff between the two and making sure that we've ported over any changes of note. The actual porting of changes will be covered by distinct issues:

• Port LRU improvements from 'old salsa' to salsa 2022 crate #365