Reject strings containing characters that are not allowed in ident

[dtolnay]

The following should fail to compile.

mashup! {
    m["x"] = a "+" b;
}

[softprops]

I figured this may generalize to something that also solves #14 but for now the other is simple enough to implement quickly on its own. Dropping the ident spec for reference here in case someone else doesn't pick this up before I do.