Symbolic testing

[g-r-a-n-t]

This PR includes the following:

• basic groundwork for invariant support
• invariants that verify the std::lib

Invariants in Fe are similar to tests. The difference being that invaraints are executed symbolically instead of concretely.

example:

#invariant
fn de_morgan_true(a: bool, b: bool) {
    spec::given_true(a and b)
    spec::assert_true(not ((not a) or (not b)))
}

#invariant
fn de_morgan_false(a: bool, b: bool) {
    spec::given_false(a and b)
    spec::assert_false(not ((not a) or (not b)))
}

In the example given above, we provide the invariants with two arguments: a and b. These variables do not have concrete values, so we need to evaluate the invariant symbolically. This is done using kevm.