Implement Process.cwd (#41)

* Implement `Process.cwd`
* Update `os` test suite

src/module/os.c

@@ -1,4 +1,5 @@
 #include "os.h"
+#include "uv.h"
 #include "wren.h"
 
 #if __APPLE__
@@ -68,4 +69,19 @@ void processAllArguments(WrenVM* vm)
     wrenSetSlotString(vm, 1, args[i]);
     wrenInsertInList(vm, 0, -1, 1);
   }
-}
+}
+
+void processCwd(WrenVM* vm)
+{
+  wrenEnsureSlots(vm, 1);
+
+  char buffer[PATH_MAX * 4];
+  size_t length = sizeof(buffer);
+  if (uv_cwd(buffer, &length) != 0)
+  {
+    wrenSetSlotString(vm, 0, "Cannot get current working directory.");
+    return wrenAbortFiber(vm, 0);
+  }
+
+  wrenSetSlotString(vm, 0, buffer);
+}

src/module/os.wren

@@ -10,4 +10,5 @@ class Process {
   static arguments { allArguments[2..-1] }
 
   foreign static allArguments
+  foreign static cwd
 }

src/module/os.wren.inc

@@ -12,4 +12,5 @@ static const char* osModuleSource =
 "  static arguments { allArguments[2..-1] }\n"
 "\n"
 "  foreign static allArguments\n"
+"  foreign static cwd\n"
 "}\n";