Handle nested columns

[roeap]

We need to handle nested columns in various locations, data skipping being an obvious one.

We should come up with a robust and simple design that is capable of handling column mapping and other special scenarios as well.

Known problems to solve:

• Make column names a first-class concept (not just strings) #422
• Store an actual path in ColumnName #443
• Project to nested columns in StructType::project #353
• Define FFI API for passing nested column names #423
• Support data skipping on nested columns #442
• Choose an official Display format for ColumnName #444

[nicklan]

@roeap can you clarify what's not properly handled today? afaict here we will traverse into nested columns.

So is there something more general you're talking about here?