util: pretty-print SIMD types

Before:

    Bool32x4 {}
    Float32x4 {}
    Int32x4 {}
    // etc.

After:

    Bool32x4 [ false, false, false, false ]
    Float32x4 [ NaN, NaN, NaN, NaN ]
    Int32x4 [ 0, 0, 0, 0 ]
    // etc.

PR-URL: https://github.com/nodejs/node/pull/6917


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>