Various performance improvements to pathname

[glebm]

1. Avoids unnecessary String allocations.
2. Reserve Array capacity when possible.
3. Avoid Char in favour of u8 when possible.
4. .ok().unwrap -> .unwrap.

Travis benchmark:

Before: https://travis-ci.org/danielpclark/faster_path/jobs/352457728

64-bit ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-linux]
64-bit rustc 1.24.1 (d3ae9a9e0 2018-02-27)
architecture: x86_64-unknown-linux-gnu
--------------------------------------------------------------------------------
Performance change for allocate, instead of new, is 77.5%
Performance change for absolute? is 88.5%
Performance change for add_trailing_separator is 39.3%
Performance change for basename is 1.8%
Performance change for children is 43.5%
Performance change for children_compat is -56.7%
Performance change for chop_basename is 78.0%
Performance change for cleanpath_aggressive is 92.0%
Performance change for cleanpath_conservative is 91.7%
Performance change for del_trailing_separator is 85.0%
Performance change for directory? is 2.0%
Performance change for dirname is 26.5%
Performance change for entries is 53.0%
Performance change for entries_compat is -102.6%
Performance change for extname is 66.4%
Performance change for has_trailing_separator? is 84.2%
Performance change for join is 71.9%
Performance change for plus is 86.6%
Performance change for relative? is 87.1%
Performance change for relative_path_from is 84.7%

After:

Performance change for allocate, instead of new, is 76.1%
Performance change for absolute? is 95.7%
Performance change for add_trailing_separator is 50.8%
Performance change for basename is 7.8%
Performance change for children is 34.6%
Performance change for children_compat is -37.4%
Performance change for chop_basename is 82.4%
Performance change for cleanpath_aggressive is 93.4%
Performance change for cleanpath_conservative is 93.1%
Performance change for del_trailing_separator is 89.5%
Performance change for directory? is 17.6%
Performance change for dirname is 52.8%
Performance change for entries is 27.6%
Performance change for entries_compat is -99.9%
Performance change for extname is 65.5%
Performance change for has_trailing_separator? is 90.3%
Performance change for join is 67.0%
Performance change for plus is 87.7%
Performance change for relative? is 93.6%
Performance change for relative_path_from is 91.0%

[coveralls]

Coverage remained the same at 93.023% when pulling f3d6f1e57f6005f7a3fad82eedb6509ccfaf0c30 on glebm:various-perf into 0257714cc7e696215eb6166620f87bf67b1b23eb on danielpclark:master.

[coveralls]

Coverage remained the same at 93.023% when pulling f3d6f1e57f6005f7a3fad82eedb6509ccfaf0c30 on glebm:various-perf into 0257714cc7e696215eb6166620f87bf67b1b23eb on danielpclark:master.