JIRA: https://issues.apache.org/jira/browse/SPARK-8207

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6721 from viirya/expr_bin and squashes the following commits:

07e1c8f [Liang-Chi Hsieh] Remove AbstractUnaryMathExpression and let BIN inherit UnaryExpression.
0677f1a [Liang-Chi Hsieh] For comments.
cf62b95 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
0cf20f2 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
dea9c12 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
d4f4774 [Liang-Chi Hsieh] Add @ignore_unicode_prefix.
7a0196f [Liang-Chi Hsieh] Fix python style.
ac2bacd [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
a0a2d0f [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
4cb764d [Liang-Chi Hsieh] For comments.
0f78682 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
c0c3197 [Liang-Chi Hsieh] Add bin to FunctionRegistry.
824f761 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into expr_bin
50e0c3b [Liang-Chi Hsieh] Add math function bin(a: long): string.