defmacro/g! and HyObject#startswith

Currently, defmacro/g! doesn't respond well when it comes across a
HyObject that doesn't respond to the instance method startswith (e.g.
HyInteger, HyFloat, etc.). This updates defmacro/g! to be a little
safer when searching for the gensyms it needs to create.
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]
Fix hy2py output on Windows/Python 3

If there are Unicode symbols in the hy2py output (as in
tests/native_test/language.hy) and the user is on Windows, and
the user is using Python3 and the user doesn't have an appropriate
codepage in the terminal for Unicode, then hy2py will fail at
encoding the output.

This fix makes sure that if encoding fails, the bytes output is shown
instead of throwing an exception and failing completely.  This also
allows the hy2py tests to pass on Windows.

If the user does activate an appropriate codepage, for example,

chcp 65001

then the Unicode output will show correctly.

Converted printing code to small function to eliminate some duplicate code
Merge pull request #602 from agentultra/shadow-builtins

Add shadow.hy to core
Merge branch 'master' into pr/616
Merge branch 'master' into pr/593