[Relay] todo in text format

[MarisaKirisame]

0: pattern matching/adt/fatal/ref for both pretty printing/parser
1: doc.h do not have a layout engine - right now it will emit the same string regardless of screen width/remaining space
2: indentation is off - nested let look incredibly broken
3: every tree - style program will still make a graph var for everything, so it is impossible to display code that resemble sml
4: src/error.cc often do not display error message for unknown reason
5: more principle solution for dealing with literal
6: to do attrs in alphaequal extensibly
7: handle floating point equality