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.

lib/system.nim

@@ -710,7 +710,7 @@ proc `&` * (x, y: string): string {.
 proc `&` * (x: char, y: string): string {.
   magic: "ConStrStr", noSideEffect, merge.}
   ## is the `concatenation operator`. It concatenates `x` and `y`.
-  
+
 # implementation note: These must all have the same magic value "ConStrStr" so
 # that the merge optimization works properly. 
 
@@ -895,7 +895,12 @@ type # these work for most platforms:
   PFloat64* = ptr Float64 ## an alias for ``ptr float64``
   PInt64* = ptr Int64 ## an alias for ``ptr int64``
   PInt32* = ptr Int32 ## an alias for ``ptr int32``
-  
+
+type TOptional*[T] = object
+  case hasValue* : bool
+  of true: value*: T
+  of false: nil
+ 
 proc toFloat*(i: int): float {.
   magic: "ToFloat", noSideEffect, importc: "toFloat".}
   ## converts an integer `i` into a ``float``. If the conversion