Missing/misspelled variable name recommendations

This is the equivalent of PR #1776 but for variables instead of keywords. 

I've moved some of the reusable recommendation code from `namespace.py` to `utils/match.py` in a new `FuzzyMatcher` class. Trying to balance code duplication and complexity in this refactoring was tricky, so criticism/ideas are welcome.

There are a lot of special cases for variable naming, including accessing variable indexes, accessing extended variables, and accessing nested variables. I have only partially handled these special cases - recommendations for misspellings of the base variable name will generally work, but anything more complex will not work, and the recommendations will not include the index or extended variable syntax. 

I reworded some of the missing variable error messages, just to make them subjectively sound better. For example, `Non-existing variable '%s'` is now `Variable '%s' not found`. This breaks many tests throughout `atest`, but in this PR I have not fixed those tests yet. I'll fix them if this rewording/rephrasing is approved. 

For example recommendations, please see the new test cases I've added.
Variables that only differ with `s` like `candidate(s)` and `norm_candidate(s)` are a bit problematic because they look so similar and are easy to mistype. For that reason I often use shorter names in for loops and list comprehensions, and would have probably used something like `cand` and `norm` here. This is more like a general programming tip than a request to change this code, though.