new compiler feature: --expandMacro

tests/compilerfeatures/texpandmacro.nim

@@ -0,0 +1,18 @@
+discard """
+  cmd: "nim c --expandMacro:foo $file"
+  nimout: '''Hint: expanded macro:
+echo ["injected echo"]
+var x = 4 [ExpandMacro]
+'''
+  output: '''injected echo'''
+"""
+
+import macros
+
+macro foo(x: untyped): untyped =
+  result = quote do:
+    echo "injected echo"
+    `x`
+
+foo:
+  var x = 4