Lexer is too eager to treat - as unary negation

[jruderman]

fn main() {
    log 3-2;
}

b.rs:2:9: 2:11 error: expected ';' or '}' after expression but found -2

[brson]

Oops, didn't notice that f26e770 included a test.

[brson]

marijn: do you want to leave this open until it's solved better (as indicated in your comments)?

[marijnh]

Yeah, my hack is not a proper solution (doesn't work for, for example x-0xFFFFFFFFu, unlikely as that is to occur). Someone will implement constant folding for our constants. Once we have that, negative literals can be removed from the lexer again, and the code that parses literal patterns can just have a special case for unary -.