Implement Bad Idol Inheritance

[mkeeter]

This PR adds a minimal inheritance implementation, which is just enough to fix our one problem child. The problem child is, of course, cpu-seq.idol – which has different operations on Cosmo versus Gimlet.

The challenge here is that both client and server still need to agree on operations. The implementation is deliberately limited to make it hard to screw up. It allows exactly one layer of inheritance: an idol file may declare a base member, which must define a BaseInterface(..) object. You can't use the BaseInterface as an Idol interface, removing one possible failure mode. In our motivating example, you must explicitly select either cpu-seq.cosmo.idol or cpu-seq.gimlet.idol (e.g. with feature flags).

We serialize the merged file into the .idolatry section of the generated code, so Humility sees all the operations (in the correct order).