Replace a couple of pir op uses with nqp ops.

Just some assorted ones where we already have the NQP op defined.
Add an nqp:: op API for working with SCs.

It replaces a rather non-uniform mix of v-table calls, method calls
and pirops. Probably there are a few more needed, but this covers the
majority.
Update bootstrap so we can use the SC ops in NQP.
Start using the nqp:: ops for SC handling.
Start updating SC tests to use new ops.