Create map entries for ebbs and values as they are defined, but leave ebb and
value operands unresolved on instructions as they are parsed. Instruction
operands can refer to ebbs and values that may not have been defined yet.
Don't infer or check result types yet.
Avoid gathering too much code in repr.rs.
The `entities` module contains entity reference types, and the `instructions`
module contains instruction opcodes and formats.
libctonfile -> libreader.
This library will only provide .cton file reading/parsing services which are
not needed after deployment.
Code for writing .cton files lives in the main cretonne library because it is
fairly small, and because it is useful for extracting test cases from a
deployed library.
