ZIR-177: Progress towards keccak codegen

[shkoo]

• Add //zirgen/Main:gen_zirgen that compiles .zir and splits out generated files that are split out instead of all in one big file
• Split out a bunch of zirgen driver.cpp to ease reuse by gen_zirgen
• Add zirgen support to build_circuit and bootstrap utility
• //zirgen/circuit filegroup target to build all circuits for faster cargo bootstrap build time
• Standardize --output-dir command line option for building circuits target directory
• Add a literal handler to codegen options to allow configuration of how literals are handled without having the codegen emitter depend on all the dialects
• Change EDSL to separate out layouts for different buffers
• Remove zkpLayoutCompat code generation option
• Remove fallbackEmitLiteral in favor of literal handler syntax
• Migrate more generators to use the literal handlers
• Remove no longer used literal-creation macros
• Remove special "context argument" types that have to be passed around everywhere; this is now configured per language per operation by add{Func,Call}ContextArg.
• generated code now exposes API via buffers instead of layouts
• Unconstrain arguments on StepFuncOp, Validity*FuncOp so we can add buffers to them
• New LowerStepsFunc pass which converts all functions to StepFuncOps; in the future we should consider whether we want to keep the difference between back and exec functions.
• New BuffersToArgs pass which transforms zstruct.get_buffer calls to function arguments
• CodegenEmitter support for emitting to multiple files with the same emitter, avoiding duplicates of e.g. typedefs
• Upgrade risc0 cargo reference to new revision on pending branch for layout changes

[tzerrell]

Your other reviewers might be able to give more informed opinions, but to me at least this looks good!