removed toolz, potential bugfixes, perfomance improvements, and more

[jorenham]

I noticed that this repo was being used by some big performance-focused projects (e.g. pymc), so I figured I'd take a look at the code. So, apparantly, this is the result of me "looking" at the code 🤷🏻‍♂️ :

• removed the need for toolz, by rewriting match.ordering using a (faster) dict comprehension
• alleged performance improvement in util._toposort
• alleged performance improvement in the variable.Var instance check
• potential bugfixes regarding a couple of == comparison of type's
• potential bugfix in case an immutable mapping is passed to core.assoc
• avoid sorting dict items on python>=3.7 in utils.freeze
• infinite-loop detection in util.transitive_get
• some pep8 fixes, related to dictionary/tuple constructors
• removed some redundant code from within try: blocks
• trimmed some if statements

... and I'm sure I'm forgetting some stuff 😅

[brandonwillard]

Thanks for the contribution! I'm pretty busy right now, so I might not get a chance to review this in the next few days, but the changes sound great, so I definitely do want to review it sooner than later.
In general, breaking each distinct change/improvement down into its own commit (or even PR) will help speed up the review a lot. If that's possible to do in the meantime, feel free to rebase this as much as you want. (I don't know if you've already done that, but it's worth mentioning just in case.)

[jorenham]

@brandonwillard Except for a few (reasonable) exceptions, each commit is limited to the changes of one function.