suggestion to improve lean_assert

Hi Leo, what do you think about this change?

Here are two examples:
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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
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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
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