Add a Platform class.

- Rename the "process" module to "os".
- Add Platform to it.
- Static "name" method.
- Static "isPosix" method.
- Docs and tests!

doc/site/modules/os/platform.markdown

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+^title Platform Class
+
+The Platform class exposes basic information about the operating system Wren is
+running on top of.
+
+## Static Methods
+
+### **name**
+
+The name of the platform. This roughly describes the operating system, and is
+usually one of:
+
+* "iOS"
+* "Linux"
+* "OS X"
+* "POSIX"
+* "Unix"
+* "Windows"
+
+If Wren was compiled for an unknown operating system, returns "Unknown".
+
+### **isPosix**
+
+Returns `true` if the host operating system is known to support the POSIX
+standard. This is true for Linux and other Unices, as well as the various Apple
+operating systems.