Move parse_expression to Parser.unsafe_parse_expression

- Add Parser#string
- Add Parser#unsafe_parse_expression
- Add private Parser#parse_expression
- Remove ParseContext.parse_expression
- Remove Tag.parse_expression
- Condition.parse_expression now takes a parser as argument

lib/liquid/parser.rb

@@ -72,9 +72,9 @@ module Liquid
       end
     end
 
+
     def expression_node
-      expr = expression
-      Expression.parse(expr, @ss, @cache)
+      parse_expression(expression)
     end
 
     def argument
@@ -104,5 +104,17 @@ module Liquid
       end
       str
     end
+
+    # Assumes safe input. For cases where you need the string.
+    # Don't use this unless you're sure about what you're doing.
+    def unsafe_parse_expression(markup)
+      parse_expression(markup)
+    end
+
+    private
+
+    def parse_expression(markup)
+      Expression.parse(markup, @ss, @cache)
+    end
   end
 end