Resolve dependent SCs table.
Set up the __6MODEL_CORE__ SC.
Stub STables and objects at deserialization start.
Fill out STable and object deserialization logic.

This also adds the REPR deserialize functions for some representations
but others (especially the rather more complex P6opaque) are to come.
This gets a knowhow with a method in to deserialize well enough to be
able to call the method, at least.
Twiddle deserialization code-gen a little.
Handle deserialization of KnowHOWAttribute.

Turns out that with this, we can now deserialize nqp-mo.pm's blob
successfully!
Enable emitting deserialize code by default.
Update ROADMAP.
implements nqp::join

Implements nqp::join and the get_str functionality of P6intInstance and
P6numInstance.
implements nqp::split
implements nqp::sprintf
implements nqp::ord
implements nqp::index
implements nqp::rindex
implements cmp_i, cmp_n, and cmp_s