[Summer] Add refinement type support for equations

[jlvargasme]

The let rec keyword is parsed to a list of equation expressions. However, the equation handler equation_desc on zparser.mly has no support for refinement types and the following example does not compile:

let rec (a, b) : btype {box(expression)} = body in output

For reference, take a look at the simple_type and type_expression implementation for refinement types on zparser.mly. We need to add a similar pattern match construct for equations.

[Easier task] - Add a pattern match for refinement types on the parse equation handler. This will enable refinement type annotation for let rec expressions.

[jchen-cs]

Being done by @SharmPranjal , #37