Implement LDA Operation

Implement LDA Operation

The LDA operation reads the value from a memory location given by the
instruction, and loads it to rA. Implementing this required fleshing out
the address calculations, fixing bugs in the `Word::as_field_spec()`
method, and fixing up visibilities of various structs. Pulling the
FieldSpec out of a Word being used as an Instruction was also necessary.

Work to have addressing respect the index offset is still necessary, but
we can update state with registers now!

This desperately needs convenience methods for testing so I can take
human readable opcodes and convert them to Words more easily.