Automatic formatting for the seq! macro

[jaybosamiya]

Fairly straightforward implementation of the seq! macro by reusing the comma-delimited-exprs. When everything within a seq fits on a single line, it places it on single line, otherwise each element gets its own line.

CC @mmcloughlin because of #42

[jaybosamiya]

Automatic merging upon approval enabled. This didn't need any snapshot updates for vstd, so should be safe to release without coordinating with Verus vstd.

[mmcloughlin]

Awesome, thank you for this!

[parno]

Generally looks good to me. In this bit:

 attr* ~ seq_str ~ bang_str ~ !"=" ~

why is the !"=" needed?

[jaybosamiya]

Hmm, I am not sure it is actually necessary for this spot; that sort of thing was necessary for the non-specialized macro invocations to prevent it from taking up a token tree in the case of someone did something like foo != bar (where the = would be a valid token_tree, iirc), but for the specific case of macros like calc and seq, we don't actually need that, since if the { does not show up, then it is not going to attempt a token_tree check there anyways. I'll push a commit shortly that removes that from both calc and seq

[parno]

Cool, thanks for looking into that.