basename: Improve performance and style

[glebm]

More idiomatic, slightly faster, extracted from #66

$  WITH_REGRESSION=1 rake pbench
Pinch-bench (Pbench) by Daniel P. Clark
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64-bit ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
64-bit rustc 1.9.0 (e4e8b6668 2016-05-18)
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Performance change for allocate, instead of new, is 80.1%
Performance change for absolute? is 92.8%
Performance change for add_trailing_separator is 55.3%
Performance change for basename is 6.3%
Performance change for chop_basename is 26.6%
Performance change for directory? is 32.6%
Performance change for extname is 51.1%
Performance change for has_trailing_separator? is 52.1%
Performance change for blank? (verses strip.empty?) is -93.9%
Performance change for relative? is 92.2%
Started with run options --seed 51653

[danielpclark]

Awesome!