Better code handling in ?eval

[technetos]

Prevent users from passing code containing fn main to play.rust-lang.org and inform them about the error.

[technetos]

This has some degree of vulnerability to false positives.... but I think it's safe enough.

Yeah its problematic if someone puts in code that happens to contain fn main and its not intended to be run as fn main() { // their code } but actually part of // their code.