Don't have keywords in the lexer and parser.

Instead of recognizing "function" as a keyword, simply match it as a
context-sensitive keyword in the parser outside functions.

filetests/parser/keywords.cton

@@ -0,0 +1,5 @@
+test cat
+
+; 'function' is not a keyword, and can be used as the name of a function too.
+function function() {}
+; check: function function()