Exclude build metadata from equality checking.

Build metadata is already excluded from precedence checking in line with
the spec. For consistency and providing strict total ordering for
Version, build metadata should also be ignored in Eq impl.

Closes #12438

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #12454 : omasanori/rust/semver-eq, r=alexcrichton

Closes #12438