nicer alignment of MathConst arrays

this should fix issue #8427. This change results in

    julia> c = MathConst[π, e, γ, catalan, φ]
    5-element Array{MathConst{sym},1}:
           π = 3.1415926535897...
           e = 2.7182818284590...
           γ = 0.5772156649015...
     catalan = 0.9159655941772...
           φ = 1.6180339887498...