Convert SourceMap scan to binary search

[dtolnay]

Fixes #492.

// [dependencies]
// proc-macro2 = { features = ["span-locations"] }

fn main() {
    let mut spans = Vec::new();
    for _ in 0..20000 {
        for tt in ". . . . . . . .".parse::<proc_macro2::TokenStream>().unwrap() {
            spans.push(tt.span());
        }
    }
    let begin = std::time::Instant::now();
    for span in spans {
        let _ = span.start();
    }
    println!("{:?}", begin.elapsed());
}

Before: 1402ms
After: 8ms