Krivine and SECD Machine

This module implements two interpreters for the functional language defined so far,

Krivine Machine (in closure form) that implements Call by Name semantics for function calls
- Given an expression e, environment E to evaluate e in, and the type for each binding in E as a list of (variable name, type), the machine first checks that the bindings in E are properly typed. Then, it packages e and its environment E into a closure and evaluates the closure starting with an empty stack (see the krv_mc function in a5_krivine.ml)
```
val krv_mc: closure -> stack -> answer (* Helper for krivine_machine *)
val krivine_machine: expr -> environment -> environment bindings' types -> answer
```
SECD Machine that implements Call by Value semantics for function calls
- Given an expression e, environment E to evaluate e in, and the type for each binding in E as a list of (variable name, type), the machine first checks that the bindings in E are properly typed. Then, it compiles the expression e into a compiled list of opcodes in Reverse Polish Notation and evaluates this compiled list using E and starting with an empty stack and empty dump (see the secd function in a5_secd.ml)
```
val compile: expr -> opcode list
val secd: stack -> environment -> opcode list -> dump -> answer (* Helper for secd_machine *)
val secd_machine: expr -> environment -> environment bindings' types -> answer
```