This CL exposes the constructor function, defines type related
information, and implements value type semantics.
It also refactors test/mjsunit/samevalue.js to test SameValue and SameValueZero.

TEST=test/mjsunit/harmony/simd.js, test/cctest/test-simd.cc

LOG=Y
BUG=v8:4124

Committed: https://crrev.com/e5ed3bee99807c502fa7d7a367ec401e16d3f773
Cr-Commit-Position: refs/heads/master@{#29689}

Review URL: https://codereview.chromium.org/1219943002

Cr-Commit-Position: refs/heads/master@{#29712}