Testing framework: improvement on random generation and module PBT

type:added

For LIGO developers

This MR improves a bit the handling of property-based testing, as now it allows generators of small values. It also implements the PBT module in the testlib.

Changelog details:

A new module Test.PBT

This new module is introduced to do simple property-based testing. It's composed of:

• val gen : 'a pbt_gen: gives a generator for type 'a (must be annotated).
• val gen_small : 'a pbt_gen: : gives a small generator for type 'a (must be annotated).
• val make_test : 'a pbt_gen -> ('a -> bool) -> 'a pbt_test: creates a test from a generator and a property.
• val run : 'a pbt_test -> nat -> 'a pbt_result: runs a test for a given number of tries. The introduced types are:
• type 'a pbt_gen: represents a generator for a type.
• type 'a pbt_test: represents a test for a type.
• type 'a pbt_result = Success | Fail of 'a: represents the result of running a test.