Avoid gathering too much code in repr.rs.
The `entities` module contains entity reference types, and the `instructions`
module contains instruction opcodes and formats.
A function owns instructions and extended basic blocks. References to these
entities are implemented as opaque structs indexing into the functions internal
tables. This avoids fighting Rust's ownership checking and it also makes
references 4 bytes on all platforms.
SSA values are identified similarly, but with an optimization for the first
value produced by an instruction. Very few instructions will produce more than
one value, and there is an extended value table for those.
