Make DSLX auto-formatter about xls HOT 3 CLOSED

Note we may want to re-open or file a new issue for just "protobuf to DSLX conversion produces really long literals in a single line" -- we could do something temporary before we have a full auto formatting solution of any kind that a) assumes infinite line length and b) just causes appropriate newlines-each-struct-field-with-indent/dedent to happen when the serialized single-line version is beyond a certain length (say 80 chars). cc @RobSpringer will leave up to you how to track that one best.

from xls.

Note that most DSLX "language tests" have auto-formatting enabled as of eb08413 -- there are about four .x files in the repo that cannot be successfully auto-formatted now, will try to knock those out next week and have std.x be auto-formatted (with a test that it remains auto-formatted).

WIP binary is dslx_fmt https://github.com/google/xls/blob/d445fc4f61bd3569c309c4d8b962145ad83efe0c/xls/dslx/dslx_fmt.cc

from xls.

I'd consider it fairly complete now -- notably most remaining holes are due to the fact comments can go in the middle of constructs at arbitrary spots. To handle that large space of possibilities we could "mark" comments from the original source text runs as being formatted properly and give an error return code if there exist any unhandled comments from the original text. But I think I can largely claim this is complete, even given the few comment-formatting escapes and some remaining 100-char escapes it mostly does what we're looking for.

from xls.