Implement getCurrentPid

[yutotakano]

Change

This PR adds a new exported function in System.Process: getCurrentPid.

This calls getCurrentProcessId from System.Win32.Process on Windows, and getProcessID from System.Posix.Process on POSIX systems.

Rationale

This seems like an oversight considering getPid is implemented already. I sometimes require the current running program's process ID. Currently, I have to do the CPP conditional logic myself every time, which is a pain. Putting this in process seems natural.

Furthermore, my understanding of process is that it is a unified interface to System.Win32.Process and System.Posix.Process. As both modules provide a function to "get the currently executing process id", I feel it makes sense to have a cross-platform wrapper function.

Remaining Issues

The @since field has been left as a TODO since I'm not sure what version this will be merged in.

Tests

I could not come up with a method to test the PR. Unlike with the test for getPid, there's nothing to compare the value against, I think. It's worked from my past experiences in personal projects.

I wasn't able to ensure a stable setup to test to begin with, as running the existing test suite stack test on the Windows version of Stack failed due to several reasons (getPid failing, and detatch_console failing). Any feedback or insight is therefore appreciated.

[snoyberg]

Thanks for the PR and the catch about the unstable stack test. I've pushed some commits to move over to GitHub Actions and temporarily disable the broken tests. Can you rebase against master?

[yutotakano]

I've confirmed the remaining tests now pass perfectly on both my Windows and Linux Haskell setups. I've also rebased and force-pushed the changes. Thanks for the quick reply!

[yutotakano]

Huh, didn't mean to tag the user, sorry SInCE!

[snoyberg]

Thank you!