Change compiler to write vectors as []/~ #2725
Description
As a temporary stepping stone in vector reform (Issue #2653), we are making the compiler write all of its vector literals using the postfix /~ notation.
It is an uglification, but I have already done it, and @graydon thinks this will make future changes to a permanent vector syntax easier to accomplish mechanically. Merging it into the tree now keeps me from having to go through constant merge hell in the interim.
