Add support for Windows (for some utilities)

[Arcterus]

Not sure if we really care about this, so I'm making it a pull request. I have essentially abstracted RawFd into RawObject, which is a RawFd on Unix and a RawHandle on Windows (I may add support for RawSocket as well, as I am thinking about implementing UtilRead and UtilWrite for TcpStream).

Note that a few tests still fail for cat and head on Windows. It may just have been because git replaced the line endings for the fixtures though.

[Arcterus]

Note that this is only failing due to cargo fmt. I’d like feedback before I fix that issue.

[codecov[bot]]

Codecov Report

Merging #35 into master will decrease coverage by 0.41%. The diff coverage is 2.2%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage    15.5%   15.08%   -0.42%     
==========================================
  Files          36       41       +5     
  Lines        3592     3698     +106     
  Branches      925      926       +1     
==========================================
+ Hits          557      558       +1     
- Misses       2766     2870     +104     
- Partials      269      270       +1

Impacted Files	Coverage Δ
src/util.rs	57.77% <ø> (+12.39%)	:arrow_up:
src/posix/sh/ast.rs	0% <0%> (ø)	:arrow_up:
src/posix/chmod/mod.rs	35.52% <0%> (ø)	:arrow_up:
src/posix/sh/builtin/cd.rs	0% <0%> (ø)	:arrow_up:
src/posix/sh/env.rs	0% <0%> (ø)	:arrow_up:
src/posix/sh/command.rs	0% <0%> (ø)	:arrow_up:
src/posix/sh/types/scoped_map.rs	0% <0%> (ø)
src/posix/echo/mod.rs	57.4% <0%> (-1.86%)	:arrow_down:
src/posix/sh/types/scoped_array.rs	0% <0%> (ø)
src/posix/sh/mod.rs	0% <0%> (ø)	:arrow_up:
... and 22 more

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[Arcterus]

This is temporarily broken on Windows due to changes I've made without access to a Windows laptop.

[Arcterus]

If no one objects I am inclined to just merge this (or close this pull request and just push my code to master) as it no longer is just changes related to adding Windows support.

[mssun]

I'm OK with the Windows support.