tables work in 'const' sections; echo supports 'nil' strings; minor cleanups

tests/vm/tconsttable.nim

@@ -0,0 +1,19 @@
+discard """
+  output: '''is
+finally
+nice!'''
+"""
+
+import tables
+
+const
+  foo = {"ah": "finally", "this": "is", "possible.": "nice!"}.toTable()
+
+# protect against overly smart compiler:
+var x = "this"
+
+echo foo[x]
+x = "ah"
+echo foo[x]
+x = "possible."
+echo foo[x]