Release cargo/src/cargo/util/shell_escape.rs independently

[ghost]

I'd love to be able to use shell escapes in contexts other than Cargo.

I'm using a a copy of an old version of that file in Thrussh, when doing an ssh exec request, which is a remote command without opening a shell.

How about a shell_escape crate, which would be used by Cargo instead of this file?

[alexcrichton]

Seems fine to me! I may not get around to this soon but if someone else would like to do so I wouldn't mind adding a dep to Cargo.

[sfackler]

@alexcrichton Should this live in this repo or can I just yank it into a new one?

[alexcrichton]

@sfackler be my guest!