add support for lambda expressions

[newville]

this adds support for lambda expressions.

[codecov]

Codecov Report

❌ Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.3%. Comparing base (f9d6613) to head (2009d73).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
asteval/asteval.py	95.2%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #144   +/-   ##
======================================
  Coverage    95.3%   95.3%           
======================================
  Files           4       4           
  Lines        2149    2171   +22     
======================================
+ Hits         2049    2071   +22     
  Misses        100     100           

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