Add shadow.hy to core

This new core module allows us to shadow the builtin Python operators so
they may be passed to sequence functions that expect functions:

    => (map / [1 2 3 4 5])
    [1.0, 0.5, 0.3333333333333333, 0.25]
Merge pull request #602 from agentultra/shadow-builtins

Add shadow.hy to core