use a global IronMeta instance

[notwaldorf]

Use a singleton IronMeta instance instead of creating one for each element.

I've done a global search, and validatorType was never used. Also, it didn't have an observer and was only used in ready, so it's unlikely it ever worked correctly (I think it was intended to be a config time property, before we knew how to document private properties). Similarly for the _validatorMeta (which already looked private-y). I've added notes to them not to be used.

I've also cached the _validator in a computed property, rather than a getter, since the getter gets called in validate(), which could be on every key stroke (for auto-validating inputs). I added a unit test for this.

Performance improvement:

I ran tattoo on this branch, and these are the results. Both failures are unrelated to this PR:

Tests for: repos/polymer status: FAILED
Tests for: repos/carbon-storage status: FAILED
82 / 84 tests passed. 23 skipped.

[notwaldorf]

@bicknellr ping

[bicknellr]

If you're ok with the breaking-ness of this change, LGTM.

[notwaldorf]

For future readers: we couldn't find any users broken by this change, and the code, as it is before this PR, is old (in the sense that it's written before we had a code style for "private", config-only variables) and incorrect (the variables had no listeners, so changing them after creation time did not actually have the effect you wanted). The alternative to not shipping this change is that elements using this behaviour are significantly more expensive than they need to be.

In the very unlikely case this PR will break you, please ping us and we will help with that.