[Relay] parser/pretty printer roundtripping

[MarisaKirisame]

This PR add support for roundtripping relay code from/to string.
Right now, it support roundtripping all the defined network, and I think this is a good time to wrap up and merge.
A few things to discuss:
Doc does not accept string input with newline/tab any more, as it will interfere with the layout algorithm that will land in the future.
The printing strategy is ugly for anfed code. It will print a variable, then graph that variable.
Similarly, there should be inline ability for simple expression such as a + b + c, instead of always allocate graph var.
FuncOp is highly redundant. Can we get this information with tvm?
I copied the josn pretty printer so it use Doc Style.
since ppl do assert alpha_equal all the time, I build it into alpha_equal to give better error message.

Future Work:
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: deprecate the old json pretty printer?
5: src/error.cc often do not display error message for unknown reason
6: more principle solution for dealing with literal
7: to do attrs in alphaequal extensibly
8: handle floating point equality

Tired and Sleepy. M.K. out.

[tqchen]

let us keep the base64 and json in the src. The main reason is that we do not want to expose these APIs as public API, but rather internal ones. Likely keeping these files in the original location won't affect anything in the parser. As we only use them via save/load json

[tqchen]

Thanks @MarisaKirisame this PR is now merged