Moved the parseAST magics to evals.nim

Added string interpolation helper routines in parserutils

Added a proof-of-concept string interpolation user-land macros (currently, only as a test case):
  the interpolated expression could either be transformed to
  concat("literal string ", $(interpolated), " end") or
  "literal string $1 end" % [$(interpolated)]

Added a very initial definition of Optional[T] generic type

A new overload of ParseIdent was added in hope to get around the fact that the 
old one doesn't work correctly in macros, but the problem persists.

tests/accept/run/tstringinterp.nim

@@ -0,0 +1,82 @@
+discard """
+  file: "tstringinterp.nim"
+  output: "Hello Alice \$ 64 | Hello Bob, 10"
+"""
+
+import macros, parseutils, strutils
+
+proc concat(strings: openarray[string]) : string =
+  result = newString(0)
+  for s in items(strings): result.add(s)
+
+# This will run though the intee
+template ProcessInterpolations(e: expr) =
+  var 
+    s = e[1].strVal
+    stringStart = 0
+ 
+  for i in interpolatedFragments(s):
+    var leadingString = s[stringStart..i.interpStart-1]
+    var interpolatedExpr = s[i.exprStart..i.exprEnd]
+
+    addString(leadingString)
+
+    var interpTargetAst = parseExpr("$(x)")
+    interpTargetAst[1][0] = parseExpr(interpolatedExpr)
+    addExpr(interpTargetAst)
+    
+    stringStart = i.interpEnd + 1
+
+  if stringStart != s.len:
+    var endingString = s[stringStart..s.len]
+    addString(endingString)
+
+macro formatStyleInterpolation(e: expr): expr =
+  var 
+    formatString = ""
+    arrayNode = newNimNode(nnkBracket)
+    idx = 1
+
+  proc addString(s: string) =
+    formatString.add(s)
+
+  proc addExpr(e: expr) =
+    arrayNode.add(e)
+    formatString.add("$" & $(idx))
+    inc idx
+    
+  ProcessInterpolations(e)
+
+  result = parseExpr("\"x\" % [y]")
+  result[1].strVal = formatString
+  result[2] = arrayNode
+
+macro concatStyleInterpolation(e: expr): expr =
+  var args : seq[PNimrodNode]
+  newSeq(args, 0)
+
+  proc addString(s: string)  = args.add(newStrLitNode(s))
+  proc addExpr(e: expr)      = args.add(e)
+
+  ProcessInterpolations(e)
+
+  result = newCall("concat", args)
+
+###
+
+proc sum(a, b, c: int): int =
+  return (a + b + c)
+
+var 
+  alice = "Alice"
+  bob = "Bob"
+  a = 10
+  b = 20
+  c = 34
+
+var
+  s1 = concatStyleInterpolation"Hello ${alice} \$ ${sum (a, b, c)}"
+  s2 = formatStyleInterpolation"Hello ${bob}, ${sum (alice.len, bob.len, 2)}"
+
+write(stdout, s1 & " | " & s2)
+