ENH: allow in-line expression assignment with df.eval

TST: tests for local name overlaps

ENH: moved assign to visit_Assign from visit_Module
DOC: update docs in enhancedperf.rst
TST: addtional tests for multiple assignment, targets
ENH: add target to Scope, use instead of resolvers
Merge pull request #5343 from jreback/assign_eval

ENH: allow in-line expression assignment with df.eval