Win32 cwd

[roblabla]

Here's my first attempt at implementing cwd for window, mostly based on the code of PSUtil.

Fixes #105

It works by parsing the PEB data structure found in each process. This data structure, along with the APIs used to acquire it, are unstable and, according to microsoft, the APIs may change. In practice, a significant amount of software is built on top of this API, and as such it is unlikely to be broken. If it does break, I expect microsoft to provide a stable API to acquire this information, like they did for the CommandLine.

I tested this with various combination on intel (x86 and x64) CPUs:

• Host: 32-bit, Heim: 32-bit, Target: 32-bit
• Host: 64-bit, Heim: 64-bit, Target: 64-bit
• Host: 64-bit, Heim: 64-bit, Target: 32-bit
• Host: 64-bit, Heim: 32-bit, Target: 32-bit

I did not implement this under ARM, as it requires using a fairly new API. We'd need to do a runtime check to see if the API is present, I'm not sure what the best way to do this is.
Furthermore, getting a 64-bit process' CWD from a 32-bit Heim is not implemented, as it requires relying on even more unstable APIs, and I don't personally need it.

[coveralls]

Coverage decreased (-5.8%) to 31.284% when pulling 9aae14e on roblabla:win32-cwd into e22e235 on heim-rs:master.

[svartalf]

Heads up: I know this PR was kinda abandoned, my apologies about that. I'll try to check as much as possible in a next neek.

[svartalf]

Alright, it is really unfair from me to keep this thing in the backlog for that long; I thought that it would be better to merge it as is and mark Process::cwd for Windows as an "unstable" function somehow (probably just in the documentation, since we don't really have any better mechanisms?).

Further bug reports (if there will be any) will help us reiterate on current functionality.

Thank you, @roblabla !