suggestion to improve lean_assert

Hi Leo, what do you think about this change?

Here are two examples:

    LEAN ASSERTION VIOLATION
    File: /Users/soonhok/work/lean/src/tests/util/numerics/mpbq.cpp
    Line: 264
    root_upper(n9, x9, 3) == false
    Arguments
    root_upper(n9, x9, 3) : bool := true
    false : bool := false

    LEAN ASSERTION VIOLATION
    File: /Users/soonhok/work/lean/src/tests/util/numerics/mpbq.cpp
    Line: 267
    n10 == mpbq(1, 0)
    Arguments
    n10 : lean::mpbq := 3/2
    mpbq(1, 0) : lean::mpbq := 1