aw-query: Add preprocessing and refcounting

[johan-bjareholt]

A very unsophisticated way to refcount.

TODO:

• Lots of TODOs in code
• Value is cloned even if variable is overwritten
• Run more benchmarks

[codecov]

Codecov Report

Merging #183 into master will decrease coverage by 8.70%.
The diff coverage is 19.35%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   59.07%   50.36%   -8.71%     
==========================================
  Files          58       60       +2     
  Lines        7707     8146     +439     
  Branches     2311     1537     -774     
==========================================
- Hits         4553     4103     -450     
- Misses       3104     4031     +927     
+ Partials       50       12      -38     

Impacted Files	Coverage Δ
aw-query/src/preprocess.rs	0.00% <0.00%> (ø)
aw-query/src/varenv.rs	0.00% <0.00%> (ø)
aw-query/src/lib.rs	27.50% <20.00%> (-20.33%)	⬇️
aw-query/src/interpret.rs	59.90% <42.85%> (-20.62%)	⬇️
aw-query/tests/query.rs	69.58% <55.55%> (-22.06%)	⬇️
aw-query/src/functions.rs	91.69% <100.00%> (-0.29%)	⬇️
aw-query/src/lexer.rs	33.89% <0.00%> (-57.07%)	⬇️
aw-query/src/parser.rs	27.45% <0.00%> (-18.78%)	⬇️
... and 6 more

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