feat(library/tactic/boolean_tactics): avoid unnecessary Let expression in proof terms

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/printer): memory access violation when printing contexts

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/has_free_vars): return false for null expression

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/tactic/goal): null hypothesis being added by to_goal

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/tactic/goal): to_goal way of handling context_entries of the form (name, domain, body) where domain is null, and body is a proof term

This commit fixes a problem exposed by t13.lean.
It has a theorem of the form:

Theorem T1 (A B : Bool) : A /\ B -> B /\ A :=
     fun assumption : A /\ B,
          let lemma1 := (show A by auto),
              lemma2 := (show B by auto)
          in (show B /\ A by auto)

When to_goal creates a goal for the metavariable associated with (show B /\ A by auto) it receives a context and proposition of the form

 [ A : Bool, B : Bool, assumption : A /\ B, lemma1 := Conjunct1 assumption, lemma2 := Conjunct2 assumption ] |- B /\ A

The context_entries "lemma1 := Conjunct1 assumption" and "lemma2 := Conjunct2 assumption" do not have a domain (aka type).
Before this commit, to_goal would simply replace and references to "lemma1" and "lemma2" in "B /\ A" with their definitions.
Note that, "B /\ A" does not contain references to "lemma1" and "lemma2". Then, the following goal is created
     A : Bool, B : Bool, assumption : A /\ B |- B /\ A
That is, the lemmas are not available when solving B /\ A.
Thus, the tactic auto produced the following (weird) proof for T1, where the lemmas are computed but not used.

    Theorem T1 (A B : Bool) (assumption : A ∧ B) : B ∧ A :=
            let lemma1 := Conjunct1 assumption,
                lemma2 := Conjunct2 assumption
            in Conj (Conjunct2 assumption) (Conjunct1 assumption)

This commit fixed that. It computes the types of "Conjunct1 assumption" and "Conjunct2 assumption", and creates the goal
     A : Bool, B : Bool, assumption : A /\ B, lemma1 : A, lemma2 : B |- B /\ A

After this commit, the proof for theorem T1 is

Theorem T1 (A B : Bool) (assumption : A ∧ B) : B ∧ A :=
    let lemma1 := Conjunct1 assumption,
        lemma2 := Conjunct2 assumption
    in Conj lemma2 lemma1

as expected.

Finally, this example suggests that the encoding

Theorem T1 (A B : Bool) : A /\ B -> B /\ A :=
     fun assumption : A /\ B,
          let lemma1 : A := (by auto),
              lemma2 : B := (by auto)
          in (show B /\ A by auto)

is more efficient than

Theorem T1 (A B : Bool) : A /\ B -> B /\ A :=
     fun assumption : A /\ B,
          let lemma1 := (show A by auto),
              lemma2 := (show B by auto)
          in (show B /\ A by auto)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lean): update expected result to reflect recent changes

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(examples/lean): new tactic example

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>