Prettyprinter

[clyben]

Add a pretty printer which either output to rust macro or let-binded egglog. Pretty printer could take a string of egglog output and parse it under current schema to RcExpr, or an RcExpr of program, output the rust macro or let-binded egglog. Add runmode PrettyPrint and OptimizedPrettyPrint, which output the rust macro form of original program or optimized program. To make rust macro look better copy it to rust file and "cargo fmt". Though you need to add ".with_arg_types(..)" and "add_ctx()" to make it runnable.

[yihozhang]

action items for this PR after a discussion:

• add caveats that map_* may violate RcExpr's invariant.
• document behaviors/limitations of the current pretty printer (inlining parallel!, sharing failure when having different contexts)
• Ignore all assumptions when pretty printing to Rust
  • user will add their own top-level types and assumptions