Deterministic cycle keys

[jg-rp]

This pull request fixes #1519 by offering an alternative method for generating context keys for the cycle tag when using "simple syntax".

Rather than using a string representation of objects returned from parse_expression(), we combine the object's class name and input argument, separated by a hyphen.

For example, given a identifier that gets parsed to a Liquid::VariableLookup, instead of a key containing something like "<Liquid::VariableLookup:0x00005632631bc858>", which changes with every occurrence of {% cycle %}, we would get "VariableLookup-x". Where x is the input argument.

Extra care has been taken to handle Liquid::C::Expression objects. Hence the use of object.respond_to?(:evaluate) and the reason for generating key strings at the same time as parsing each expression.