Requires and ensures clauses with descriptions

[feliperodri]

Similar to how __CPROVER_assert(bool cond, char *msg) accepts a string that shows up in the terminal output,
we could modify the front end to allow users to name ensures and requires clauses in function contracts with an optional string. For example,

int foo(int *arr, size_t size)
__CPROVER_requires(size > 0 , "some text")
__CPROVER_ensures(__CPROVER_return_value == 1 , "some text")
{
}

[martin-cs]

Sounds good. I guess the challenge is how to do it without breaking backwards compatibility.