Use String#unpack for better extraction of words from segments

[postmodern]

String#unpack should be faster and provide more format options (such as floats). Segments will also have to be padded with \0 bytes.

[postmodern]

Did an experiment with having #each_word read each byte into a String buffer then call String#unpack and it's slightly slower than just doing bitwise math:

String#unpack

                                        user     system      total        real
Hexdump.dump (output)              10.810911   0.000000  10.810911 ( 10.869495)
Hexdump.dump width=256 (output)     9.483426   0.003832   9.487258 (  9.543299)
Hexdump.dump ascii=true (output)   10.606700   0.000000  10.606700 ( 10.641688)
Hexdump.dump word_size=2 (output)  19.317735   0.000010  19.317745 ( 19.392457)
Hexdump.dump word_size=4 (output)  18.739036   0.000013  18.739049 ( 18.826742)
Hexdump.dump word_size=8 (output)  10.603023   0.000004  10.603027 ( 10.643707)
Hexdump.dump (block)                5.508398   0.000000   5.508398 (  5.524796)
Hexdump.dump width=256 (block)      5.540716   0.000000   5.540716 (  5.557970)
Hexdump.dump ascii=true (block)     6.221936   0.000000   6.221936 (  6.245208)
Hexdump.dump word_size=2 (block)   15.909723   0.000000  15.909723 ( 15.967684)
Hexdump.dump word_size=4 (block)   15.541515   0.000000  15.541515 ( 15.605455)
Hexdump.dump word_size=8 (block)    8.204565   0.000858   8.205423 (  8.232888)

Bitshifting

                                        user     system      total        real
Hexdump.dump (output)              10.582241   0.000055  10.582296 ( 10.626448)
Hexdump.dump width=256 (output)     8.836189   0.008962   8.845151 (  8.877264)
Hexdump.dump ascii=true (output)   11.071560   0.000000  11.071560 ( 11.115871)
Hexdump.dump word_size=2 (output)  16.192893   0.002944  16.195837 ( 16.283254)
Hexdump.dump word_size=4 (output)  13.824119   0.000981  13.825100 ( 13.883479)
Hexdump.dump word_size=8 (output)   7.523130   0.000001   7.523131 (  7.554442)
Hexdump.dump (block)                5.918965   0.000000   5.918965 (  5.951641)
Hexdump.dump width=256 (block)      5.788696   0.000000   5.788696 (  5.811731)
Hexdump.dump ascii=true (block)     6.425460   0.000000   6.425460 (  6.444785)
Hexdump.dump word_size=2 (block)   12.501186   0.002875  12.504061 ( 12.588385)
Hexdump.dump word_size=4 (block)   10.756808   0.000000  10.756808 ( 10.798557)
Hexdump.dump word_size=8 (block)    5.307631   0.000000   5.307631 (  5.324436)