new genAst as replacement for quote do (#17426)

* new `macros.genAst`: fixes all issues with `quote do`
* add changelog entry
* add workaround for https://github.com/nim-lang/Nim/issues/2465#issuecomment-511076669
* add test for #9607
* add kNoExposeLocalInjects option
* add test case for nested application of genAst
* genAst: automatically call newLit when needed
* allow skipping `{}`: genAst: foo
* add test that shows this fixes #11986
* add examples showing mixin; add examples showing passing types, macros, templates
* move to std/genasts
* improve docs

tests/stdlib/mgenast.nim

@@ -0,0 +1,53 @@
+import std/genasts
+import std/macros
+
+# Using a enum instead of, say, int, to make apparent potential bugs related to
+# forgetting converting to NimNode via newLit, see bug #9607
+
+type Foo* = enum kfoo0, kfoo1, kfoo2, kfoo3, kfoo4
+
+proc myLocalPriv(): auto = kfoo1
+proc myLocalPriv2(): auto = kfoo1
+macro bindme2*(): untyped =
+  genAst: myLocalPriv()
+macro bindme3*(): untyped =
+  ## myLocalPriv must be captured explicitly
+  genAstOpt({kDirtyTemplate}, myLocalPriv): myLocalPriv()
+
+macro bindme4*(): untyped =
+  ## calling this won't compile because `myLocalPriv` isn't captured
+  genAstOpt({kDirtyTemplate}): myLocalPriv()
+
+macro bindme5UseExpose*(): untyped =
+  genAst: myLocalPriv2()
+
+macro bindme5UseExposeFalse*(): untyped =
+  genAstOpt({kDirtyTemplate}): myLocalPriv2()
+
+# example from bug #7889
+from std/streams import newStringStream, readData, writeData
+
+macro bindme6UseExpose*(): untyped =
+  genAst:
+    var tst = "sometext"
+    var ss = newStringStream("anothertext")
+    writeData(ss, tst[0].addr, 2)
+    discard readData(ss, tst[0].addr, 2)
+
+macro bindme6UseExposeFalse*(): untyped =
+  ## with `kDirtyTemplate`, requires passing all referenced symbols
+  ## which can be tedious
+  genAstOpt({kDirtyTemplate}, newStringStream, writeData, readData):
+    var tst = "sometext"
+    var ss = newStringStream("anothertext")
+    writeData(ss, tst[0].addr, 2)
+    discard readData(ss, tst[0].addr, 2)
+
+
+proc locafun1(): auto = "in locafun1"
+proc locafun2(): auto = "in locafun2"
+# locafun3 in caller scope only
+macro mixinExample*(): untyped =
+  genAst:
+    mixin locafun1
+    (locafun1(), locafun2(), locafun3())