Add defined variables to dictionary

[stefnotch]

When parsing or serializing an expression, we need to get all the currently defined variables and add them to the dictionary. That way, cat := 3 followed by cat = will work correctly

[stefnotch]

Also, the := parser might have to be rewritten a little bit to do some custom parsing for the right side. Let's first see how the next MathJson version handles various things.

[stefnotch]

Test case: % := 1/100

[phcreery]

Another one is subscripts. I often use subscripts to differentiate the same variables for different domains. W_earth=m*g_earth and W_mars=m*g_mars