perf: faster Expr.eval by precompiling expressions

[tlambert03]

This PR moves the compilation of the expression to the __init__... this should make evaluating expressions up to ~30x faster (based on local testing).

It also allows passing kwargs: expr.eval(x=1, z=3) instead of expr.eval({"x": 1, "z": 3}) , though it will be slightly slower (sub-microsecond)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8289d03) to head (c2df7a2).
Report is 9 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           31        31           
  Lines         1811      1810    -1     
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- Hits          1811      1810    -1     

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