add setenv, unsetenv function to os

os/os.v

@@ -316,6 +316,20 @@ pub fn getenv(key string) string {
 	return string(s)
 }
 
+
+pub fn setenv(name string, value string, overwrite bool) int {
+  return C.setenv(name.cstr(), value.cstr(), overwrite)
+}
+
+pub fn unsetenv(name string) int {
+  return C.unsetenv(name.cstr())
+}
+
+fn exit(code int) {
+	C.exit(code)
+}
+
+
 // `file_exists` returns true if `path` exists.
 pub fn file_exists(path string) bool {
 	res := false

os/os_test.v

@@ -0,0 +1,21 @@
+import os
+
+fn test_setenv() {
+  os.setenv('foo', 'bar', true)
+  assert os.getenv('foo') == 'bar'
+  
+  // `setenv` should not set if `overwrite` is false
+  os.setenv('foo', 'bar2', false)
+  assert os.getenv('foo') == 'bar'
+  
+  // `setenv` should overwrite if `overwrite` is true
+  os.setenv('foo', 'bar2', true)
+  assert os.getenv('foo') == 'bar2'
+}
+
+fn test_unsetenv() {
+  os.setenv('foo', 'bar', true)
+  os.unsetenv('foo')
+  assert os.getenv('foo') == ''
+}
+