Speed-up path element operations & comparisons

Avoid repeated expensive operations in `STPathElement` (e.g. calls to `base_uint::isZero`) and attempt to make comparisons of path elements faster by calculating a path id and using it to avoid expensive comparisons between `base_uint` objects.

This is pending a performance review.
More uniform might be:
```
    return (mType & typeAccount) == (t.mType & typeAccount) &&
            hash_value_ == t.hash_value_ &&
            mAccountID == t.mAccountID &&
            mCurrencyID == t.mCurrencyID &&
            mIssuerID == t.mIssuerID;
```

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