Backwards arrow compound punctuator

Hi. (I read the contribution guidelines and I think I'm flagging this for you as a mistake in the spec, rather than as an upstream change. I hope this is helpful.)

rustc rejects <-: it is lexed as a single token, which isn't legal anywhere. For < followed by - one must write < - :

error: unexpected token: `<-`
 --> src/main.rs:2:22
  |
2 |     println!("{}", 4 <- 3);
  |                      ^^
  |
help: if you meant to write a comparison against a negative value, add a space in between `<` and `-`

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c2ad15c3b9cd3bc8a98f31478b3e78f4

However the spec section 2 doesn't seem to discuss this. I think this should probably be fixed by adding <- to the table of compund punctuators (2.2.8) and the table of punctuator names (2.2.13).

The impact seems very low; it just means that rustc rejects some programs that the spec regards as correct.

Locate the FLS source for spec section 2 and read the compound punctuators table in 2.2.8 alongside the punctuator names table in 2.2.13. Update both tables to account for <-, then verify that the specification consistently describes this lexical case.