slice support in system.nim; syntactic sugar for tables; cleanup of grammar/parser

tests/accept/run/tslices.nim

@@ -0,0 +1,43 @@
+discard """
+  file: "tslices.nim"
+  output: '''456456
+456456
+456456
+Zugr5nd
+'''
+"""
+
+# Test the new slices.
+
+import strutils
+
+var mystr = "Abgrund"
+mystr[..1] = "Zu"
+
+mystr[4..4] = "5"
+
+type
+  TEnum = enum e1, e2, e3, e4, e5, e6
+
+var myarr: array[TEnum, int] = [1, 2, 3, 4, 5, 6]
+myarr[e1..e3] = myarr[e4..e6]
+myarr[..e3] = myarr[e4..e6]
+
+for x in items(myarr): stdout.write(x)
+echo()
+
+var myarr2: array[0..5, int] = [1, 2, 3, 4, 5, 6]
+myarr2[0..2] = myarr2[3..5]
+
+for x in items(myarr2): stdout.write(x)
+echo()
+
+
+var myseq = @[1, 2, 3, 4, 5, 6]
+myseq[0..2] = myseq[-3.. -1]
+
+for x in items(myseq): stdout.write(x)
+echo()
+
+echo mystr
+