Add record types to IR

A record type is a tuple with named fields. I'm guessing it will be possible to mostly just treat records as special tuples. Tupleindex instruction would work the same. Might need a new instruction kRecord which is analogous to kTuple but with field names. Similarly structured record types and tuple types would not be compatible.

The motivation is to enable richer types (structs) in the emitted SystemVerilog.