Pegs AST read access (#8050)

* Make PEG AST nodes readable from outside the module.

* Added a test module for the pegs stdlib module.

* Edited changelog.

* Renamed ``sons`` iterator to ``items``, added ``pairs``, inlined both.

* Updated entry and moved it to the right category.

tests/stdlib/tpegs.nim

@@ -0,0 +1,78 @@
+discard """
+  output: '''
+pkNonTerminal: Sum @(2, 3)
+  pkSequence: (Product (('+' / '-') Product)*)
+    pkNonTerminal: Product @(3, 7)
+      pkSequence: (Value (('*' / '/') Value)*)
+        pkNonTerminal: Value @(4, 5)
+          pkOrderedChoice: (([0-9] [0-9]*) / ('(' Expr ')'))
+            pkSequence: ([0-9] [0-9]*)
+              pkCharChoice: [0-9]
+              pkGreedyRepSet: [0-9]*
+            pkSequence: ('(' Expr ')')
+              pkChar: '('
+              pkNonTerminal: Expr @(1, 4)
+                pkNonTerminal: Sum @(2, 3)
+              pkChar: ')'
+        pkGreedyRep: (('*' / '/') Value)*
+          pkSequence: (('*' / '/') Value)
+            pkOrderedChoice: ('*' / '/')
+              pkChar: '*'
+              pkChar: '/'
+            pkNonTerminal: Value @(4, 5)
+    pkGreedyRep: (('+' / '-') Product)*
+      pkSequence: (('+' / '-') Product)
+        pkOrderedChoice: ('+' / '-')
+          pkChar: '+'
+          pkChar: '-'
+        pkNonTerminal: Product @(3, 7)
+'''
+"""
+
+import strutils, streams
+import pegs
+
+const
+  indent = "  "
+
+let
+  pegSrc = """
+Expr <- Sum
+Sum <- Product (('+' / '-') Product)*
+Product <- Value (('*' / '/') Value)*
+Value <- [0-9]+ / '(' Expr ')'
+  """
+  pegAst: Peg = pegSrc.peg
+
+var
+  outp = newStringStream()
+  processed: seq[string] = @[]
+
+proc prt(outp: Stream, kind: PegKind, s: string; level: int = 0) =
+  outp.writeLine indent.repeat(level) & "$1: $2" % [$kind, s]
+
+proc recLoop(p: Peg, level: int = 0) =
+  case p.kind
+  of pkEmpty..pkWhitespace:
+    discard
+  of pkTerminal, pkTerminalIgnoreCase, pkTerminalIgnoreStyle:
+    outp.prt(p.kind, $p, level)
+  of pkChar, pkGreedyRepChar:
+    outp.prt(p.kind, $p, level)
+  of pkCharChoice, pkGreedyRepSet:
+    outp.prt(p.kind, $p, level)
+  of pkNonTerminal:
+    outp.prt(p.kind,
+      "$1 @($3, $4)" % [p.nt.name, $p.nt.rule.kind, $p.nt.line, $p.nt.col], level)
+    if not(p.nt.name in processed):
+      processed.add p.nt.name
+      p.nt.rule.recLoop level+1
+  of pkBackRef..pkBackRefIgnoreStyle:
+    outp.prt(p.kind, $p, level)
+  else:
+    outp.prt(p.kind, $p, level)
+    for s in items(p):
+      s.recLoop level+1
+
+pegAst.recLoop
+echo outp.data